feat: make core pages local-first offline

frontend/lib/shared/api/config_api.dart

@@ -1,5 +1,9 @@
+import 'dart:convert';
+
 import 'package:flutter/material.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/session_store.dart';
 
 class BrandConfig {
@@ -41,6 +45,13 @@ class AvatarItem {
       defaultName = j['defaultName'] as String,
       speechTic = j['speechTic'] as String? ?? '',
       imageUrl = j['imageUrl'] as String?;
+
+  Map<String, dynamic> toJson() => {
+    'key': key,
+    'defaultName': defaultName,
+    'speechTic': speechTic,
+    'imageUrl': imageUrl,
+  };
 }
 
 class PersonaItem {
@@ -50,6 +61,13 @@ class PersonaItem {
       name = j['name'] as String,
       description = j['description'] as String? ?? '',
       sampleLine = j['sampleLine'] as String? ?? '';
+
+  Map<String, dynamic> toJson() => {
+    'key': key,
+    'name': name,
+    'description': description,
+    'sampleLine': sampleLine,
+  };
 }
 
 class CompanionDisplay {
@@ -145,16 +163,59 @@ class PublicConfigApi {
 
   static Future<List<AvatarItem>> avatars() async {
     final res = await _dio.get('/api/public/avatars');
-    return (res.data as List)
+    final values = (res.data as List)
         .map((e) => AvatarItem.fromJson(e as Map<String, dynamic>))
         .toList();
+    await _cacheCatalog(
+      _avatarCacheKey,
+      values.map((e) => e.toJson()).toList(),
+    );
+    return values;
   }
 
   static Future<List<PersonaItem>> personas() async {
     final res = await _dio.get('/api/public/personas');
-    return (res.data as List)
+    final values = (res.data as List)
         .map((e) => PersonaItem.fromJson(e as Map<String, dynamic>))
         .toList();
+    await _cacheCatalog(
+      _personaCacheKey,
+      values.map((e) => e.toJson()).toList(),
+    );
+    return values;
+  }
+
+  static Future<List<AvatarItem>> cachedAvatars() async =>
+      (await _cachedCatalog(_avatarCacheKey)).map(AvatarItem.fromJson).toList();
+
+  static Future<List<PersonaItem>> cachedPersonas() async =>
+      (await _cachedCatalog(
+        _personaCacheKey,
+      )).map(PersonaItem.fromJson).toList();
+
+  static String get _avatarCacheKey =>
+      'public_avatars_${BackendIdentity.scope}';
+  static String get _personaCacheKey =>
+      'public_personas_${BackendIdentity.scope}';
+
+  static Future<void> _cacheCatalog(
+    String key,
+    List<Map<String, dynamic>> values,
+  ) async {
+    await (await SharedPreferences.getInstance()).setString(
+      key,
+      jsonEncode(values),
+    );
+  }
+
+  static Future<List<Map<String, dynamic>>> _cachedCatalog(String key) async {
+    final value = (await SharedPreferences.getInstance()).getString(key);
+    if (value == null) return const [];
+    try {
+      return (jsonDecode(value) as List).cast<Map<String, dynamic>>().toList();
+    } catch (_) {
+      return const [];
+    }
   }
 
   static String? _normalizeName(String? value) {