feat: make core pages local-first offline

frontend/lib/shared/api/auth_api.dart

@@ -1,5 +1,9 @@
+import 'dart:convert';
+
 import 'package:dio/dio.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import 'package:miaoji_zhang/shared/api/api_client.dart';
+import 'package:miaoji_zhang/shared/api/backend_identity.dart';
 import 'package:miaoji_zhang/shared/services/current_ledger_store.dart';
 import 'package:miaoji_zhang/shared/services/local_export_service.dart';
 import 'package:miaoji_zhang/shared/services/session_store.dart';
@@ -19,6 +23,15 @@ class AiCompanion {
       roastLevel = json['roastLevel'] as int,
       stickerFrequency = json['stickerFrequency'] as int,
       proactiveLevel = json['proactiveLevel'] as int;
+
+  Map<String, dynamic> toJson() => {
+    'avatarKey': avatarKey,
+    'personaKey': personaKey,
+    'customName': customName,
+    'roastLevel': roastLevel,
+    'stickerFrequency': stickerFrequency,
+    'proactiveLevel': proactiveLevel,
+  };
 }
 
 class UserProfile {
@@ -49,8 +62,9 @@ class UserProfile {
     required this.nickname,
     required this.appMode,
     required this.onboardingDone,
+    this.aiCompanion,
     this.aiEnabled = false,
-  }) : aiCompanion = null;
+  });
 }
 
 class AuthLoginResult {
@@ -120,6 +134,20 @@ class AuthApi {
     }
   }
 
+  static Future<AiCompanion?> cachedCompanion() async {
+    final userId = SessionStore.instance.userId;
+    if (userId == null) return null;
+    final value = (await SharedPreferences.getInstance()).getString(
+      _companionCacheKey(userId),
+    );
+    if (value == null) return null;
+    try {
+      return AiCompanion.fromJson(jsonDecode(value) as Map<String, dynamic>);
+    } catch (_) {
+      return null;
+    }
+  }
+
   static Future<UserProfile> completeOnboarding({
     required String appMode,
     required String avatarKey,
@@ -272,13 +300,24 @@ class AuthApi {
     );
   }
 
-  static Future<void> _cacheProfile(UserProfile profile) =>
-      SessionStore.instance.activateAccount(
-        userId: profile.userId,
-        username: profile.username,
-        nickname: profile.nickname,
-        appMode: profile.appMode,
-        onboardingDone: profile.onboardingDone,
-        aiEnabled: profile.aiEnabled,
+  static Future<void> _cacheProfile(UserProfile profile) async {
+    await SessionStore.instance.activateAccount(
+      userId: profile.userId,
+      username: profile.username,
+      nickname: profile.nickname,
+      appMode: profile.appMode,
+      onboardingDone: profile.onboardingDone,
+      aiEnabled: profile.aiEnabled,
+    );
+    final companion = profile.aiCompanion;
+    if (companion != null) {
+      await (await SharedPreferences.getInstance()).setString(
+        _companionCacheKey(profile.userId),
+        jsonEncode(companion.toJson()),
       );
+    }
+  }
+
+  static String _companionCacheKey(int userId) =>
+      'ai_companion_${BackendIdentity.scope}_$userId';
 }