feat(client): 登录体验 — 记住邮箱 + 密码显隐 + 7天免登陆

1. 记住邮箱:登录页预填上次邮箱(TokenStore.saveLastEmail,登录/注册成功时存,
   退出登录不清除)。
2. 密码显隐:登录/注册密码框加眼睛按钮切换明文(PangolinIcons.eye/eyeOff)。
3. 7天免登陆:RootFlow 改为响应 authProvider——启动时有有效会话直接进主界面;
   AuthNotifier.refresh() 在服务端拒绝(refresh 过期/超 7 天)时 logout 回登录页,
   网络错误则保留会话。会话恢复期间显示极简启动屏。

flutter analyze 0 error;114 tests passed。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/main.dart

@@ -7,8 +7,10 @@ import 'pangolin_theme.dart';
 import 'services/token_store.dart';
 import 'shell/home_shell.dart';
 import 'state/app_providers.dart';
+import 'state/auth_provider.dart';
 import 'widgets/auth_screen.dart';
 import 'widgets/onboarding_screen.dart';
+import 'widgets/pangolin_logo.dart';
 
 void main() => runApp(const ProviderScope(child: PangolinApp()));
 
@@ -29,9 +31,8 @@ class PangolinApp extends ConsumerWidget {
   }
 }
 
-/// 应用阶段:登录 → 引导 → 主应用
-enum AppStage { auth, onboarding, app }
-
+/// 应用根流转:启动时若已有有效会话(7 天内 refresh 有效)直接进主界面,
+/// 免登陆;会话失效则回登录页。引导页仅首次注册/登录后看一次。
 class RootFlow extends ConsumerStatefulWidget {
   const RootFlow({super.key});
   @override
@@ -39,32 +40,43 @@ class RootFlow extends ConsumerStatefulWidget {
 }
 
 class _RootFlowState extends ConsumerState<RootFlow> {
-  AppStage stage = AppStage.auth;
   final _store = const TokenStore();
+  bool _forceOnboarding = false;
 
-  // 登录后:引导只首次看过一次;之后直接进主界面。
+  // 登录/注册成功后:未看过引导才进引导,否则直接主界面。
   Future<void> _afterAuth() async {
     final onboarded = await _store.isOnboarded();
     if (!mounted) return;
-    setState(() => stage = onboarded ? AppStage.app : AppStage.onboarding);
+    setState(() => _forceOnboarding = !onboarded);
   }
 
   Future<void> _afterOnboarding() async {
     await _store.markOnboarded();
     if (!mounted) return;
-    setState(() => stage = AppStage.app);
+    setState(() => _forceOnboarding = false);
   }
 
   @override
   Widget build(BuildContext context) {
     final t = ref.watch(appTextProvider);
-    switch (stage) {
-      case AppStage.auth:
-        return AuthScreen(t: t, onDone: () { _afterAuth(); });
-      case AppStage.onboarding:
-        return OnboardingScreen(t: t, onDone: () { _afterOnboarding(); });
-      case AppStage.app:
-        return const HomeShell();
+    final auth = ref.watch(authProvider);
+
+    if (auth.isLoading) return const _Splash();
+    if (!auth.isLoggedIn) {
+      return AuthScreen(t: t, onDone: () { _afterAuth(); });
     }
+    if (_forceOnboarding) {
+      return OnboardingScreen(t: t, onDone: () { _afterOnboarding(); });
+    }
+    return const HomeShell();
   }
 }
+
+/// 会话恢复期间的极简启动屏(避免登录页一闪而过)。
+class _Splash extends StatelessWidget {
+  const _Splash();
+  @override
+  Widget build(BuildContext context) => const Scaffold(
+        body: Center(child: PangolinMark(size: 56)),
+      );
+}