Validate stale chat requests, show info instead of auto-dismiss

- Add validateIncomingRequest() — checks session status with backend
- Home screen validates before showing sheet (on resume + listener)
- IncomingRequestSheet shows "cancelled/accepted by other" message
  instead of silently dismissing when request becomes stale

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mitra_app/lib/features/home/home_screen.dart

@@ -32,7 +32,13 @@ class _HomeScreenState extends ConsumerState<HomeScreen> with WidgetsBindingObse
     if (state == AppLifecycleState.resumed) {
       final chatState = ref.read(chatRequestProvider);
       if (chatState is ChatRequestIncomingData) {
-        _showIncomingRequest(chatState.sessionId);
+        // Validate the request is still pending before showing
+        ref.read(chatRequestProvider.notifier).validateIncomingRequest().then((_) {
+          final current = ref.read(chatRequestProvider);
+          if (current is ChatRequestIncomingData) {
+            _showIncomingRequest(current.sessionId);
+          }
+        });
       }
     }
   }
@@ -65,7 +71,13 @@ class _HomeScreenState extends ConsumerState<HomeScreen> with WidgetsBindingObse
     // Listen for incoming chat requests
     ref.listen(chatRequestProvider, (prev, next) {
       if (next is ChatRequestIncomingData) {
-        _showIncomingRequest(next.sessionId);
+        // Validate request is still pending before showing sheet
+        ref.read(chatRequestProvider.notifier).validateIncomingRequest().then((_) {
+          final current = ref.read(chatRequestProvider);
+          if (current is ChatRequestIncomingData) {
+            _showIncomingRequest(current.sessionId);
+          }
+        });
       } else if (next is ChatRequestAcceptedData) {
         final session = next.session;
         final sessionId = session['session_id'] as String? ?? session['id'] as String;