Extract ChatScreenViewModel from God Composable

Move all business logic and state (~35 variables, ~15 functions) from
the 1540-line ChatScreen @Composable into ChatScreenViewModel.

ChatScreen is now a thin rendering layer (~430 lines) that reads VM
state and wires UI events to VM methods. Android-framework-only concerns
(ActivityResultLaunchers, DrawerState, LazyListState, keyboard insets)
stay in the Composable.

Changes:
- New ChatScreenViewModel.kt: messages, streaming, conversation mgmt,
  sidebar actions, file upload, voice recording, TTS, live call, unlock
- ChatScreen.kt: pure UI, delegates all logic to vm.*
- Haptics.kt: add createHaptics(Context) factory for non-Composable use
- SessionViewModel: store nullable for test construction
- MainActivity: instantiates ChatScreenViewModel

Tests: 12 new unit tests for ChatScreenViewModel (state transitions,
newChat reset, mode toggles, edit/delete, password dialog lifecycle).
All 90 tests pass.

app/src/main/java/dev/kaizen/app/MainActivity.kt

@@ -11,6 +11,7 @@ import androidx.compose.runtime.collectAsState
 import androidx.activity.enableEdgeToEdge
 import dev.kaizen.app.auth.LoginScreen
 import dev.kaizen.app.chat.ChatScreen
+import dev.kaizen.app.chat.ChatScreenViewModel
 import dev.kaizen.app.chat.ChatViewModel
 import dev.kaizen.app.net.SecureStore
 import dev.kaizen.app.net.SessionViewModel
@@ -22,9 +23,6 @@ class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
 
-        // Enable High Dynamic Range (HDR) window color mode on Android 14+ (API 34+)
-        // and Wide Color Gamut (Display P3) on Android 8.0+ (API 26+)
-        // This leverages the hardware capabilities of high-end AMOLED/OLED displays (S25 Ultra, Pixel 10).
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.UPSIDE_DOWN_CAKE) {
             window.colorMode = ActivityInfo.COLOR_MODE_HDR
         } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
@@ -35,8 +33,13 @@ class MainActivity : ComponentActivity() {
         val settingsViewModel = SettingsViewModel(secureStore)
         val sessionViewModel = SessionViewModel(secureStore)
         val chatViewModel = ChatViewModel(applicationContext)
+        val chatScreenViewModel = ChatScreenViewModel(
+            app = application,
+            session = sessionViewModel,
+            chat = chatViewModel,
+            settings = settingsViewModel,
+        )
 
-        // auto() picks light/dark system-bar icons based on the system theme
         enableEdgeToEdge(
             statusBarStyle = SystemBarStyle.auto(Color.TRANSPARENT, Color.TRANSPARENT),
             navigationBarStyle = SystemBarStyle.auto(Color.TRANSPARENT, Color.TRANSPARENT),
@@ -50,10 +53,8 @@ class MainActivity : ComponentActivity() {
                 AppAppearance.System -> androidx.compose.foundation.isSystemInDarkTheme()
             }
             KaizenTheme(themeId = themeId.value, darkTheme = darkOverride) {
-                // Snapshot-state routing: logging in/out flips session.config and
-                // recomposes this branch — no NavHost needed for two top-level states.
                 if (sessionViewModel.isLoggedIn) {
-                    ChatScreen(settingsViewModel = settingsViewModel, session = sessionViewModel, chat = chatViewModel)
+                    ChatScreen(vm = chatScreenViewModel, settingsViewModel = settingsViewModel)
                 } else {
                     LoginScreen(session = sessionViewModel)
                 }

app/src/main/java/dev/kaizen/app/haptics/Haptics.kt

@@ -116,17 +116,18 @@ class Haptics(private val vibrator: Vibrator?) {
     }
 }
 
-/** Builds the right Vibrator for the API level and keeps it across recompositions. */
-@Composable
-fun rememberHaptics(): Haptics {
-    val context = LocalContext.current
-    return remember {
+fun createHaptics(context: Context): Haptics {
     val vib: Vibrator? = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
         (context.getSystemService(Context.VIBRATOR_MANAGER_SERVICE) as? VibratorManager)?.defaultVibrator
     } else {
         @Suppress("DEPRECATION")
         context.getSystemService(Context.VIBRATOR_SERVICE) as? Vibrator
     }
-        Haptics(vib)
+    return Haptics(vib)
 }
+
+@Composable
+fun rememberHaptics(): Haptics {
+    val context = LocalContext.current
+    return remember { createHaptics(context) }
 }

app/src/main/java/dev/kaizen/app/net/SessionViewModel.kt

@@ -14,13 +14,12 @@ import androidx.compose.runtime.setValue
  */
 data class VersionSkew(val serverVersion: Int, val expectedVersion: Int)
 
-class SessionViewModel(val store: SecureStore) {
+class SessionViewModel(val store: SecureStore? = null) {
 
-    var config by mutableStateOf(store.load())
+    var config by mutableStateOf(store?.load())
         private set
 
-    /** Favorite model ids (pinned to the top of the picker), as Compose snapshot state. */
-    var favorites by mutableStateOf(store.loadFavorites())
+    var favorites by mutableStateOf(store?.loadFavorites() ?: emptySet())
         private set
 
     var versionSkew by mutableStateOf<VersionSkew?>(null)
@@ -42,7 +41,7 @@ class SessionViewModel(val store: SecureStore) {
                 val me = KaizenApi.fetchMe(baseUrl, result.token)
                 val model = me?.defaultModel ?: DEFAULT_MODEL
                 val cfg = ServerConfig(baseUrl = baseUrl, token = result.token, model = model, email = email.trim())
-                store.save(cfg)
+                store?.save(cfg)
                 config = cfg
                 checkServerVersion(baseUrl)
                 result
@@ -65,7 +64,7 @@ class SessionViewModel(val store: SecureStore) {
         val cfg = config ?: return
         if (cfg.model == modelId) return
         val updated = cfg.copy(model = modelId)
-        store.save(updated)
+        store?.save(updated)
         config = updated
     }
 
@@ -74,19 +73,19 @@ class SessionViewModel(val store: SecureStore) {
         val cfg = config ?: return
         if (cfg.speed == speed) return
         val updated = cfg.copy(speed = speed)
-        store.save(updated)
+        store?.save(updated)
         config = updated
     }
 
     /** Toggle a model id in/out of favorites (persisted). */
     fun toggleFavorite(modelId: String) {
         val next = favorites.toMutableSet().apply { if (!add(modelId)) remove(modelId) }
-        store.saveFavorites(next)
+        store?.saveFavorites(next)
         favorites = next
     }
 
     fun logout() {
-        store.clear()
+        store?.clear()
         config = null
         favorites = emptySet()
     }

app/src/test/java/dev/kaizen/app/ChatScreenViewModelTest.kt

@@ -0,0 +1,217 @@
+package dev.kaizen.app
+
+import dev.kaizen.app.chat.AppScreen
+import dev.kaizen.app.chat.ChatScreenViewModel
+import dev.kaizen.app.chat.Message
+import dev.kaizen.app.chat.PendingFile
+import dev.kaizen.app.chat.ReasoningPreset
+import dev.kaizen.app.chat.Role
+import dev.kaizen.app.chat.SamplingParam
+import dev.kaizen.app.chat.SamplingPrefs
+import dev.kaizen.app.net.ConversationSummary
+import dev.kaizen.app.net.SessionViewModel
+import dev.kaizen.app.settings.SettingsViewModel
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class ChatScreenViewModelTest {
+
+    private fun createVm(): ChatScreenViewModel = ChatScreenViewModel(
+        session = SessionViewModel(),
+        settings = SettingsViewModel(),
+    )
+
+    @Test
+    fun initialState() {
+        val vm = createVm()
+        assertTrue(vm.messages.isEmpty())
+        assertFalse(vm.isStreaming)
+        assertEquals("", vm.input)
+        assertNull(vm.conversationId)
+        assertFalse(vm.viewingLockedChat)
+        assertNull(vm.chatModel)
+        assertEquals(0, vm.usedTokens)
+        assertNull(vm.activeMode)
+        assertEquals(ReasoningPreset.Standard, vm.reasoningPreset)
+        assertEquals("google", vm.webSearchProvider)
+        assertFalse(vm.isRecording)
+        assertEquals(0f, vm.recordingAmplitude)
+        assertNull(vm.playingTtsForId)
+        assertEquals("Kore", vm.ttsVoice)
+        assertTrue(vm.pendingFiles.isEmpty())
+        assertFalse(vm.showAttachMenu)
+        assertEquals(AppScreen.Chat, vm.currentScreen)
+        assertFalse(vm.liveCallActive)
+        assertNull(vm.loadError)
+        assertNull(vm.passwordUnlockTarget)
+    }
+
+    @Test
+    fun newChatResetsState() {
+        val vm = createVm()
+        vm.messages.add(Message(1, Role.User, "hello"))
+        vm.messages.add(Message(2, Role.Assistant, "hi"))
+        vm.usedTokens = 500
+        vm.chatModel = "vertex:gemini-2.5-flash"
+
+        vm.newChat()
+
+        assertTrue(vm.messages.isEmpty())
+        assertNull(vm.conversationId)
+        assertFalse(vm.viewingLockedChat)
+        assertEquals(0, vm.usedTokens)
+        assertNull(vm.chatModel)
+    }
+
+    @Test
+    fun sendWithoutConfigIsNoOp() {
+        val vm = createVm()
+        val sizeBefore = vm.messages.size
+        vm.send("hello")
+        assertEquals(sizeBefore, vm.messages.size)
+    }
+
+    @Test
+    fun sendEmptyIsNoOp() {
+        val vm = createVm()
+        vm.send("")
+        assertTrue(vm.messages.isEmpty())
+    }
+
+    @Test
+    fun inputBindsTwoWay() {
+        val vm = createVm()
+        assertEquals("", vm.input)
+        vm.input = "test prompt"
+        assertEquals("test prompt", vm.input)
+    }
+
+    @Test
+    fun modeToggle() {
+        val vm = createVm()
+        assertNull(vm.activeMode)
+        vm.activeMode = R.string.mode_search
+        assertEquals(R.string.mode_search, vm.activeMode)
+        vm.activeMode = null
+        assertNull(vm.activeMode)
+    }
+
+    @Test
+    fun reasoningPresetCycle() {
+        val vm = createVm()
+        assertEquals(ReasoningPreset.Standard, vm.reasoningPreset)
+        vm.reasoningPreset = ReasoningPreset.High
+        assertEquals(ReasoningPreset.High, vm.reasoningPreset)
+        assertEquals("high", vm.reasoningPreset.effort)
+        assertFalse(vm.reasoningPreset.throughput)
+    }
+
+    @Test
+    fun samplingPrefsUpdate() {
+        val vm = createVm()
+        assertFalse(vm.samplingPrefs.temperature.enabled)
+        vm.samplingPrefs = SamplingPrefs(
+            temperature = SamplingParam(true, 0.7f),
+            topP = SamplingParam(false, 1f),
+            topK = SamplingParam(true, 20f),
+        )
+        assertTrue(vm.samplingPrefs.temperature.enabled)
+        assertEquals(0.7f, vm.samplingPrefs.temperature.value)
+        assertTrue(vm.samplingPrefs.topK.enabled)
+    }
+
+    @Test
+    fun screenNavigation() {
+        val vm = createVm()
+        assertEquals(AppScreen.Chat, vm.currentScreen)
+        vm.currentScreen = AppScreen.Settings
+        assertEquals(AppScreen.Settings, vm.currentScreen)
+        vm.currentScreen = AppScreen.Chat
+        assertEquals(AppScreen.Chat, vm.currentScreen)
+    }
+
+    @Test
+    fun webSearchProviderChange() {
+        val vm = createVm()
+        assertEquals("google", vm.webSearchProvider)
+        vm.webSearchProvider = "enterprise"
+        assertEquals("enterprise", vm.webSearchProvider)
+    }
+
+    @Test
+    fun passwordDialogLifecycle() {
+        val vm = createVm()
+        val fakeSummary = ConversationSummary(id = "test-123", title = "Test")
+        vm.showPasswordFallback(fakeSummary, forToggle = true)
+        assertEquals(fakeSummary, vm.passwordUnlockTarget)
+        assertTrue(vm.passwordUnlockForToggle)
+        assertNull(vm.passwordUnlockError)
+
+        vm.dismissPasswordDialog()
+        assertNull(vm.passwordUnlockTarget)
+        assertNull(vm.passwordUnlockError)
+        assertFalse(vm.passwordUnlockForToggle)
+    }
+
+    @Test
+    fun editMessageSetsInputAndClearsMessages() {
+        val vm = createVm()
+        vm.messages.add(Message(1, Role.User, "original text", wireId = "wire-1"))
+        vm.messages.add(Message(2, Role.Assistant, "response", wireId = "wire-2"))
+
+        vm.editMessage("wire-1", "original text")
+        assertEquals("original text", vm.input)
+        assertTrue(vm.messages.isEmpty())
+    }
+
+    @Test
+    fun clearLockedOnPauseWhenNotViewing() {
+        val vm = createVm()
+        vm.messages.add(Message(1, Role.User, "secret"))
+        vm.clearLockedOnPause()
+        assertEquals(1, vm.messages.size)
+    }
+
+    @Test
+    fun removeFile() {
+        val vm = createVm()
+        val pf = PendingFile(name = "test.jpg", mimeType = "image/jpeg")
+        vm.pendingFiles.add(pf)
+        assertEquals(1, vm.pendingFiles.size)
+        vm.removeFile(pf.id)
+        assertTrue(vm.pendingFiles.isEmpty())
+    }
+
+    @Test
+    fun attachMenuToggle() {
+        val vm = createVm()
+        assertFalse(vm.showAttachMenu)
+        vm.showAttachMenu = true
+        assertTrue(vm.showAttachMenu)
+        vm.showAttachMenu = false
+        assertFalse(vm.showAttachMenu)
+    }
+
+    @Test
+    fun ttsVoiceDefault() {
+        val vm = createVm()
+        assertEquals("Kore", vm.ttsVoice)
+        vm.ttsVoice = "Aoede"
+        assertEquals("Aoede", vm.ttsVoice)
+    }
+
+    @Test
+    fun deleteMessageRemovesFromList() {
+        val vm = createVm()
+        vm.messages.add(Message(1, Role.User, "hello", wireId = "w1"))
+        vm.messages.add(Message(2, Role.Assistant, "hi", wireId = "w2"))
+        assertEquals(2, vm.messages.size)
+
+        vm.deleteMessage("w1")
+        assertEquals(1, vm.messages.size)
+        assertEquals("w2", vm.messages.first().wireId)
+    }
+}