feat: make core pages local-first offline

frontend/lib/shared/api/api_client.dart

@@ -4,6 +4,8 @@ import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:miaoji_zhang/shared/api/backend_identity.dart';
 import 'package:miaoji_zhang/shared/services/session_store.dart';
 
+enum BackendAvailability { unknown, online, offline }
+
 class ApiClient {
   ApiClient._();
   static final ApiClient instance = ApiClient._();
@@ -12,6 +14,9 @@ class ApiClient {
   static const _legacyTokenKey = 'auth_token';
   static final _tokenKey = 'auth_token_${BackendIdentity.scope}';
   static final sessionExpired = ValueNotifier<int>(0);
+  static final availability = ValueNotifier<BackendAvailability>(
+    BackendAvailability.unknown,
+  );
   static bool _handlingUnauthorized = false;
 
   static const String baseUrl = BackendIdentity.baseUrl;
@@ -31,6 +36,10 @@ class ApiClient {
           )
           ..interceptors.add(
             InterceptorsWrapper(
+              onResponse: (response, handler) {
+                availability.value = BackendAvailability.online;
+                handler.next(response);
+              },
               onRequest: (options, handler) async {
                 final token = await _storage.read(key: _tokenKey);
                 if (token != null) {
@@ -39,6 +48,11 @@ class ApiClient {
                 handler.next(options);
               },
               onError: (error, handler) async {
+                if (isConnectivityError(error)) {
+                  availability.value = BackendAvailability.offline;
+                } else if (error.response != null) {
+                  availability.value = BackendAvailability.online;
+                }
                 final unauthorized = error.response?.statusCode == 401;
                 final data = error.response?.data;
                 final aiDenied =
@@ -74,6 +88,18 @@ class ApiClient {
   }
 
   Future<String?> readToken() => _storage.read(key: _tokenKey);
+
+  Future<void> probe() async {
+    try {
+      await dio.get<void>(
+        '/api/public/brand',
+        options: Options(receiveTimeout: const Duration(seconds: 10)),
+      );
+    } catch (_) {
+      // The interceptor owns the reachability state transition.
+    }
+  }
+
   Future<void> clearToken() async {
     await _storage.delete(key: _tokenKey);
     await _storage.delete(key: _legacyTokenKey);
@@ -91,8 +117,9 @@ String apiErrorMessage(Object e) {
   if (e is StateError) return e.message;
   if (e is DioException) {
     final data = e.response?.data;
-    if (data is Map && data['message'] != null)
+    if (data is Map && data['message'] != null) {
       return data['message'] as String;
+    }
     if (e.type == DioExceptionType.connectionTimeout ||
         e.type == DioExceptionType.connectionError ||
         e.type == DioExceptionType.receiveTimeout) {