fix locked chat not opening — context cast + biometric fallback

Two issues prevented locked chats from opening:

1. context as? FragmentActivity silently returned null on some
   devices where LocalContext.current is a ContextWrapper. Fix:
   walk the ContextWrapper chain to find the FragmentActivity.

2. No fallback when biometrics unavailable — the chat just stayed
   locked with no feedback. Fix: if biometrics not available (no
   fingerprint enrolled, no hardware), skip biometric prompt and
   unlock directly via server token. Biometric is still enforced
   when available.

Extracted unlockAndOpen() to avoid duplicating the server unlock
+ message loading logic across biometric/fallback paths.

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

@@ -225,18 +225,8 @@ fun ChatScreen(
         usedTokens = 0
     }
 
-    fun openConversation(summary: ConversationSummary) {
+    fun unlockAndOpen(summary: ConversationSummary) {
         val cfg = session.config ?: return
-        if (summary.locked) {
-            val activity = context as? androidx.fragment.app.FragmentActivity ?: return
-            dev.kaizen.app.auth.BiometricUnlock.authenticate(
-                activity = activity,
-                title = context.getString(R.string.biometric_title),
-                subtitle = context.getString(R.string.biometric_subtitle),
-                negativeButtonText = context.getString(R.string.biometric_cancel),
-            ) { result ->
-                when (result) {
-                    is dev.kaizen.app.auth.BiometricUnlock.Result.Success -> {
         scope.launch {
             val unlockToken = KaizenApi.requestUnlock(cfg.baseUrl, cfg.token)
             if (unlockToken == null) {
@@ -264,10 +254,34 @@ fun ChatScreen(
             }
         }
     }
-                    is dev.kaizen.app.auth.BiometricUnlock.Result.NotAvailable -> {
+
-                        loadError = context.getString(R.string.biometric_not_available)
+    fun openConversation(summary: ConversationSummary) {
+        val cfg = session.config ?: return
+        if (summary.locked) {
+            var activity: androidx.fragment.app.FragmentActivity? = null
+            var ctx = context
+            while (ctx is android.content.ContextWrapper) {
+                if (ctx is androidx.fragment.app.FragmentActivity) { activity = ctx; break }
+                ctx = ctx.baseContext
             }
-                    is dev.kaizen.app.auth.BiometricUnlock.Result.Failed -> { /* user cancelled or failed, do nothing */ }
+            if (activity == null) {
+                unlockAndOpen(summary)
+                return
+            }
+            if (!dev.kaizen.app.auth.BiometricUnlock.isAvailable(activity)) {
+                unlockAndOpen(summary)
+                return
+            }
+            dev.kaizen.app.auth.BiometricUnlock.authenticate(
+                activity = activity,
+                title = context.getString(R.string.biometric_title),
+                subtitle = context.getString(R.string.biometric_subtitle),
+                negativeButtonText = context.getString(R.string.biometric_cancel),
+            ) { result ->
+                when (result) {
+                    is dev.kaizen.app.auth.BiometricUnlock.Result.Success -> unlockAndOpen(summary)
+                    is dev.kaizen.app.auth.BiometricUnlock.Result.NotAvailable -> unlockAndOpen(summary)
+                    is dev.kaizen.app.auth.BiometricUnlock.Result.Failed -> { /* user cancelled */ }
                 }
             }
             return