fix token badge, hide pills on keyboard, reset tokens on nav

Three fixes:
1. Token badge only shown when usedTokens > 0 (not in old chats
   where we have no streaming data). Pushed to far right with
   Spacer weight. Reset to 0 on new chat and conversation switch.
2. Mode pills hidden when keyboard is open (WindowInsets.isImeVisible)
   — prevents the bottom dock from pushing too far up and overlapping
   chat content. Input field stays compact at the bottom.
3. ModelPill no longer constrained with weight(fill=false) which
   was causing layout issues.

app/src/main/java/dev/kaizen/app/chat/ChatScreen.kt

@@ -13,7 +13,10 @@ import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.ExperimentalLayoutApi
+import androidx.compose.foundation.layout.WindowInsets
 import androidx.compose.foundation.layout.imePadding
+import androidx.compose.foundation.layout.isImeVisible
 import androidx.compose.foundation.layout.navigationBarsPadding
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
@@ -219,6 +222,7 @@ fun ChatScreen(
         messages.clear()
         conversationId = null
         viewingLockedChat = false
+        usedTokens = 0
     }
 
     fun openConversation(summary: ConversationSummary) {
@@ -269,6 +273,7 @@ fun ChatScreen(
             return
         }
         viewingLockedChat = false
+        usedTokens = 0
         scope.launch {
             messages.clear()
             val cachedList = chat.messageRepo.getCached(summary.id)
@@ -655,11 +660,10 @@ fun ChatScreen(
                                 ModelPill(
                                     label = modelDisplayName(session.config?.model ?: "", models),
                                     onClick = { haptics.tick(); showModelSheet = true },
-                                    modifier = Modifier.weight(1f, fill = false),
                                 )
-                                if (usedTokens > 0 || messages.isNotEmpty()) {
+                                Spacer(Modifier.weight(1f))
+                                if (usedTokens > 0) {
                                     val ctxLen = models.find { it.id == session.config?.model }?.context_length ?: 1_000_000
-                                    Spacer(Modifier.width(8.dp))
                                     TokenBadge(used = usedTokens, contextLength = ctxLen)
                                 }
                             }
@@ -704,6 +708,9 @@ fun ChatScreen(
                                 .imePadding()
                                 .then(if (isTablet) Modifier.widthIn(max = 720.dp) else Modifier)
                         ) {
+                            @OptIn(ExperimentalLayoutApi::class)
+                            val imeVisible = WindowInsets.isImeVisible
+                            if (!imeVisible) {
                                 Spacer(Modifier.height(24.dp))
                                 ModePillsRow(
                                     activeMode = activeMode,
@@ -716,6 +723,7 @@ fun ChatScreen(
                                     samplingPrefs = samplingPrefs,
                                     onSamplingChange = { samplingPrefs = it },
                                 )
+                            }
                             Spacer(Modifier.height(10.dp))
                             ChatInput(
                                 value = input,