Preserve voice session across tab navigation

Add watchdog mechanism to ensure STT continues when switching tabs:
- Monitor STT state every 500ms and restart if session active but STT stopped
- Handle AppState changes to restart STT when app returns to foreground
- Prevents session interruption due to native audio session changes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/(tabs)/_layout.tsx

@@ -1,6 +1,6 @@
 import { Tabs } from 'expo-router';
 import React, { useCallback, useEffect, useRef } from 'react';
-import { Platform, View } from 'react-native';
+import { Platform, View, AppState, AppStateStatus } from 'react-native';
 import { Feather } from '@expo/vector-icons';
 import { useSafeAreaInsets } from 'react-native-safe-area-context';
 
@@ -143,6 +143,55 @@ export default function TabLayout() {
     }
   }, [status, sttIsListening, startListening]);
 
+  // ============================================================================
+  // TAB NAVIGATION PERSISTENCE
+  // Ensure voice session continues when user switches between tabs.
+  // The session state is in VoiceContext (root level), but STT may stop due to:
+  // 1. Native audio session changes
+  // 2. Tab unmount/remount (though tabs layout doesn't unmount)
+  // 3. AppState changes (background/foreground)
+  // ============================================================================
+
+  // Monitor and recover STT state during tab navigation
+  // If session is active but STT stopped unexpectedly, restart it
+  useEffect(() => {
+    // Check every 500ms if STT needs to be restarted
+    const intervalId = setInterval(() => {
+      // Only act if session should be active (isListening from VoiceContext)
+      // but STT is not actually listening, and we're not in speaking/processing mode
+      if (
+        sessionActiveRef.current &&
+        !sttIsListening &&
+        status !== 'speaking' &&
+        status !== 'processing'
+      ) {
+        console.log('[TabLayout] STT watchdog: restarting STT (session active but STT stopped)');
+        startListening();
+      }
+    }, 500);
+
+    return () => clearInterval(intervalId);
+  }, [sttIsListening, status, startListening]);
+
+  // Handle app state changes (background/foreground)
+  // When app comes back to foreground, restart STT if session was active
+  useEffect(() => {
+    const handleAppStateChange = (nextAppState: AppStateStatus) => {
+      if (nextAppState === 'active' && sessionActiveRef.current) {
+        // App came to foreground, give it a moment then check STT
+        setTimeout(() => {
+          if (sessionActiveRef.current && !sttIsListening && status !== 'speaking' && status !== 'processing') {
+            console.log('[TabLayout] App foregrounded - restarting STT');
+            startListening();
+          }
+        }, 300);
+      }
+    };
+
+    const subscription = AppState.addEventListener('change', handleAppStateChange);
+    return () => subscription.remove();
+  }, [sttIsListening, status, startListening]);
+
   // Handle voice FAB press - toggle listening mode
   const handleVoiceFABPress = useCallback(() => {
     if (isListening) {