Stop voice session when app goes to background

User feedback: voice session should stop when leaving app or locking screen.

Previously:
- App goes to background → session stays active (broken state)
- STT/TTS cannot work in background anyway
- Wasted battery, confusing UX

Now:
- background/inactive → automatically stops session
- Cleans up all state (STT, refs, pending transcripts)
- User must manually restart via FAB when returning

Behavior:
User locks screen → session stops → FAB becomes gray
User switches app → session stops → clean state
User returns → must press FAB to start new session

app/(tabs)/_layout.tsx:238-256

app/(tabs)/_layout.tsx

@@ -238,19 +238,27 @@ export default function TabLayout() {
   // Handle app state changes (background/foreground)
   useEffect(() => {
     const handleAppStateChange = (nextAppState: AppStateStatus) => {
-      if (nextAppState === 'active' && sessionActiveRef.current) {
-        setTimeout(() => {
-          if (sessionActiveRef.current && !sttIsListening && status !== 'processing' && status !== 'speaking') {
-            console.log('[TabLayout] App foregrounded - restarting STT');
-            safeStartSTT();
-          }
-        }, 500);
+      // When app goes to background/inactive - stop voice session
+      // STT/TTS cannot work in background, so it's pointless to keep session active
+      if ((nextAppState === 'background' || nextAppState === 'inactive') && sessionActiveRef.current) {
+        console.log('[TabLayout] App going to background - stopping voice session');
+        stopListening();
+        stopSession();
+        sessionActiveRef.current = false;
+        shouldRestartSTTRef.current = false;
+        pendingInterruptTranscriptRef.current = null;
+      }
+
+      // When app comes back to foreground - do NOT auto-restart session
+      // User must manually press FAB to start new session
+      if (nextAppState === 'active') {
+        console.log('[TabLayout] App foregrounded - session remains stopped (user must restart via FAB)');
       }
     };
 
     const subscription = AppState.addEventListener('change', handleAppStateChange);
     return () => subscription.remove();
-  }, [sttIsListening, status, safeStartSTT]);
+  }, [stopListening, stopSession]);
 
   // Handle voice FAB press - toggle listening mode
   // Must check ALL active states (listening, processing, speaking), not just isListening