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/chat/widgets/incoming_request_sheet.dart

@@ -8,6 +8,18 @@ class IncomingRequestSheet extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
+    final requestState = ref.watch(chatRequestProvider);
+
+    // Request is still active — show accept/decline
+    if (requestState is ChatRequestIncomingData) {
+      return _buildActiveRequest(context, ref);
+    }
+
+    // Request was taken by another mitra or cancelled — show info
+    return _buildStaleRequest(context);
+  }
+
+  Widget _buildActiveRequest(BuildContext context, WidgetRef ref) {
     return Container(
       padding: const EdgeInsets.all(24),
       child: Column(
@@ -52,4 +64,35 @@ class IncomingRequestSheet extends ConsumerWidget {
       ),
     );
   }
+
+  Widget _buildStaleRequest(BuildContext context) {
+    return Container(
+      padding: const EdgeInsets.all(24),
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          const Icon(Icons.info_outline, size: 48, color: Colors.orange),
+          const SizedBox(height: 16),
+          const Text(
+            'Permintaan tidak tersedia',
+            style: TextStyle(fontSize: 20, fontWeight: FontWeight.bold),
+          ),
+          const SizedBox(height: 8),
+          const Text(
+            'Permintaan ini sudah dibatalkan oleh customer atau diterima oleh Bestie lain.',
+            style: TextStyle(fontSize: 14, color: Colors.grey),
+            textAlign: TextAlign.center,
+          ),
+          const SizedBox(height: 24),
+          SizedBox(
+            width: double.infinity,
+            child: ElevatedButton(
+              onPressed: () => Navigator.of(context).pop(),
+              child: const Text('OK'),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
 }