feat(locale): 实现 App 启动时语言和时区自动设置

- 新增系统语言/时区读取工具函数
- SessionStore 扩展支持时区存储
- 启动流程自动检测并保存系统语言/时区
- 注册时传递语言/时区到后端
- 登录后从服务器同步语言/时区

apps/lib/core/auth/session_store.dart

@@ -14,6 +14,7 @@ class SessionStore {
   static const String _emailKey = 'saved_email';
   static const String _welcomeReadKey = 'has_seen_welcome_dialog';
   static const String _localeKey = 'selected_locale';
+  static const String _timezoneKey = 'selected_timezone';
 
   Future<void> saveToken(String token) async {
     await _secureStorage.write(key: _tokenKey, value: token);
@@ -66,4 +67,12 @@ class SessionStore {
   Future<String?> getLocaleTag() async {
     return _kvStore.getString(_localeKey);
   }
+
+  Future<void> saveTimezone(String timezone) async {
+    await _kvStore.setString(_timezoneKey, timezone);
+  }
+
+  Future<String?> getTimezone() async {
+    return _kvStore.getString(_timezoneKey);
+  }
 }

apps/lib/core/localization/system_locale.dart

@@ -0,0 +1,27 @@
+import 'dart:ui';
+
+Locale? resolveSystemLocale(Locale systemLocale) {
+  final lang = systemLocale.languageCode.toLowerCase();
+  final script = systemLocale.scriptCode;
+  final country = systemLocale.countryCode;
+
+  if (lang == 'en') {
+    return const Locale('en');
+  }
+
+  if (lang == 'zh') {
+    if (script == 'Hant' ||
+        country == 'TW' ||
+        country == 'HK' ||
+        country == 'MO') {
+      return const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant');
+    }
+    return const Locale('zh');
+  }
+
+  return null;
+}
+
+Locale getSystemLocale() {
+  return PlatformDispatcher.instance.locale;
+}

apps/lib/core/timezone/system_timezone.dart

@@ -0,0 +1,5 @@
+import 'package:flutter_timezone/flutter_timezone.dart';
+
+Future<String> getSystemTimezone() async {
+  return FlutterTimezone.getLocalTimezone();
+}