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/widgets/account_screens.dart

@@ -98,7 +98,7 @@ class PlansScreen extends ConsumerWidget {
     Widget body() => plansAsync.when(
           loading: () => const Center(child: Padding(padding: EdgeInsets.all(40), child: CircularProgressIndicator())),
           error: (_, __) => Center(
-              child: Padding(padding: const EdgeInsets.all(40), child: Text(t.lang == AppLang.zh ? '加载失败,请重试' : 'Failed to load, retry', style: PangolinText.body.copyWith(color: c.fg3)))),
+              child: Padding(padding: const EdgeInsets.all(40), child: Text(t.lang.loadFailedRetry, style: PangolinText.body.copyWith(color: c.fg3)))),
           data: (plans) => ListView(
             padding: const EdgeInsets.fromLTRB(20, 14, 20, 24),
             children: [
@@ -140,13 +140,8 @@ class DevicesScreen extends ConsumerWidget {
 
   // 相对时间(最后登录);null → 从未登录。
   String _rel(DateTime? ts) {
-    final zh = t.lang == AppLang.zh;
     if (ts == null) return t.devNeverLogin;
-    final d = DateTime.now().difference(ts.toLocal());
-    if (d.inMinutes < 1) return zh ? '刚刚' : 'just now';
-    if (d.inMinutes < 60) return zh ? '${d.inMinutes} 分钟前' : '${d.inMinutes} min ago';
-    if (d.inHours < 24) return zh ? '${d.inHours} 小时前' : '${d.inHours}h ago';
-    return zh ? '${d.inDays} 天前' : '${d.inDays}d ago';
+    return t.lang.relativeTime(DateTime.now().difference(ts.toLocal()));
   }
 
   @override
@@ -158,12 +153,12 @@ class DevicesScreen extends ConsumerWidget {
     Widget content() => devicesAsync.when(
           loading: () => const Center(child: Padding(padding: EdgeInsets.all(40), child: CircularProgressIndicator())),
           error: (_, __) => Center(
-              child: Padding(padding: const EdgeInsets.all(40), child: Text(t.lang == AppLang.zh ? '加载失败,请重试' : 'Failed to load, retry', style: PangolinText.body.copyWith(color: c.fg3)))),
+              child: Padding(padding: const EdgeInsets.all(40), child: Text(t.lang.loadFailedRetry, style: PangolinText.body.copyWith(color: c.fg3)))),
           data: (devices) => ListView(padding: const EdgeInsets.fromLTRB(20, 4, 20, 24), children: [
             Text(t.devicesSub, style: PangolinText.caption.copyWith(color: c.fg3, fontWeight: FontWeight.w400)),
             const SizedBox(height: 12),
             if (devices.isEmpty)
-              Text(t.lang == AppLang.zh ? '暂无已登录设备' : 'No devices yet',
+              Text(t.lang.noDevices,
                   style: PangolinText.sm.copyWith(color: c.fg3))
             else
               Container(