feat(client/i18n): B1 硬编码文案 6 语化 + B2 语言持久化

B2 持久化:localeProvider 由 StateProvider 改 StateNotifier(LocaleNotifier),
默认英文,选择存 shared_preferences(key pg_lang),重启保留;新增 appTextFor(lang)
helper 供非 Consumer 场景复用。两处切换 .state= 改 .set()。

B1 硬编码 UI 文案 6 语化(AppLangMisc 扩展,exhaustive switch 不会漏):
- 不可用 / 流媒体优化 / 节点未就绪 / 连接失败 / 加载失败 / 暂无设备 / 改邮箱
- 相对时间(刚刚/X分钟前/…)relativeTime(Duration)
- 修 auth_screen 错误消息 bug:原 _errorZh 永远显示中文(连英文用户都中招)→
  按当前语言取 messageZh/En(非中文显英文)

仍回退英文(服务端数据,需服务端多语字段):节点/套餐名(nameZh/nameEn)、
API 错误消息(messageZh/En)。flutter analyze 仅 2 个既有 withOpacity info。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/screens/account_page.dart

@@ -75,7 +75,7 @@ class AccountPage extends ConsumerWidget {
           _CustomRow(
             icon: PangolinIcons.globe,
             title: t.language,
-            trailing: _LangSwitch(lang: lang, onChange: (l) => ref.read(localeProvider.notifier).state = l),
+            trailing: _LangSwitch(lang: lang, onChange: (l) => ref.read(localeProvider.notifier).set(l)),
           ),
           Divider(height: 1, color: c.border),
           _CustomRow(

client/lib/screens/nodes_page.dart

@@ -267,7 +267,7 @@ class _NodeGridTile extends StatelessWidget {
   Widget build(BuildContext context) {
     final c = context.pangolin;
     final down = node.isDown; // 节点不可用(agent 离线):置灰 + 「不可用」+ 禁选。
-    final unavail = lang == AppLang.zh ? '不可用' : 'Unavailable';
+    final unavail = lang.unavailable;
     return Opacity(
       opacity: down ? 0.55 : 1.0,
       child: Material(

client/lib/screens/settings_page.dart

@@ -72,7 +72,7 @@ class SettingsPage extends ConsumerWidget {
           const SizedBox(height: 16),
           // 配置组
           _Card(children: [
-            _Row(title: t.language, right: _LangSwitch(lang: lang, onPick: (l) => ref.read(localeProvider.notifier).state = l)),
+            _Row(title: t.language, right: _LangSwitch(lang: lang, onPick: (l) => ref.read(localeProvider.notifier).set(l))),
             _Row(
               title: t.darkAppearance,
               sub: isDark ? t.stateOn : t.followLight,