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/auth_screen.dart

@@ -39,7 +39,7 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
   bool _sent = false;
   bool _loading = false;
   bool _pwVisible = false;
-  String? _errorZh;
+  String? _error;
 
   final _email = TextEditingController();
   final _code = TextEditingController();
@@ -90,17 +90,17 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
   // ── 认证操作（逻辑不变）──────────────────────────────────────────
 
   Future<void> _sendCode() async {
-    setState(() { _loading = true; _errorZh = null; });
+    setState(() { _loading = true; _error = null; });
     try {
       await _api.sendCode(_email.text.trim());
       if (mounted) setState(() { _sent = true; _loading = false; });
     } on AuthApiException catch (e) {
-      if (mounted) setState(() { _errorZh = e.messageZh; _loading = false; });
+      if (mounted) setState(() { _error = widget.t.lang == AppLang.zh ? e.messageZh : e.messageEn; _loading = false; });
     }
   }
 
   Future<void> _doRegister() async {
-    setState(() { _loading = true; _errorZh = null; });
+    setState(() { _loading = true; _error = null; });
     try {
       final device = (await ref.read(deviceIdentityProvider).meta()).toJson();
       final tokens = await _api.register(
@@ -113,12 +113,12 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
       await ref.read(authProvider.notifier).saveTokens(tokens);
       if (mounted) widget.onDone();
     } on AuthApiException catch (e) {
-      if (mounted) setState(() { _errorZh = e.messageZh; _loading = false; });
+      if (mounted) setState(() { _error = widget.t.lang == AppLang.zh ? e.messageZh : e.messageEn; _loading = false; });
     }
   }
 
   Future<void> _doLogin() async {
-    setState(() { _loading = true; _errorZh = null; });
+    setState(() { _loading = true; _error = null; });
     try {
       final device = (await ref.read(deviceIdentityProvider).meta()).toJson();
       final tokens = await _api.login(
@@ -132,7 +132,7 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
       ref.read(deviceLimitProvider.notifier).state = tokens.deviceLimit;
       if (mounted) widget.onDone();
     } on AuthApiException catch (e) {
-      if (mounted) setState(() { _errorZh = e.messageZh; _loading = false; });
+      if (mounted) setState(() { _error = widget.t.lang == AppLang.zh ? e.messageZh : e.messageEn; _loading = false; });
     }
   }
 
@@ -167,7 +167,7 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
         _mode = m;
         _step = 0;
         _sent = false;
-        _errorZh = null;
+        _error = null;
       });
 
   // ── UI ──────────────────────────────────────────────────────
@@ -278,7 +278,7 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
         child: Column(children: [
           _segTabs(c, t),
           const SizedBox(height: 22),
-          if (_errorZh != null) ...[
+          if (_error != null) ...[
             Container(
               padding: const EdgeInsets.symmetric(horizontal: 14, vertical: 10),
               decoration: BoxDecoration(
@@ -289,7 +289,7 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
               child: Row(children: [
                 Icon(PangolinIcons.x, size: 16, color: c.danger),
                 const SizedBox(width: 8),
-                Expanded(child: Text(_errorZh!, style: PangolinText.sm.copyWith(color: c.danger))),
+                Expanded(child: Text(_error!, style: PangolinText.sm.copyWith(color: c.danger))),
               ]),
             ),
             const SizedBox(height: 14),
@@ -457,7 +457,7 @@ class _AuthScreenState extends ConsumerState<AuthScreen>
               style: PangolinText.sm.copyWith(color: c.fg1, fontWeight: FontWeight.w600))),
           GestureDetector(
             onTap: () => setState(() => _step = 0),
-            child: Text(t.lang == AppLang.zh ? '改邮箱' : 'Change',
+            child: Text(t.lang.changeEmail,
                 style: PangolinText.caption.copyWith(color: c.accent, fontWeight: FontWeight.w600, fontSize: 13)),
           ),
         ]),