feat: 外观设置后端持久化 + 保存按钮 + 跨浏览器同步

后端:
- User 模型新增 Preferences JSON 字段 (theme + login_style)
- 新增 GET/PUT /api/v1/user/preferences (auth) 个人偏好读写
- 新增 GET /api/v1/public/login-style (public) 登录页风格公开端点
- SQLite users 表手动添加 preferences 列

前端:
- AppearanceSettings 从后端加载偏好 + 保存按钮 + dirty 状态提示
- LoginPage 先从后端公开接口获取 login_style (跨浏览器一致)
- useLogin 登录成功后从后端加载偏好并应用主题
- 偏好同时存 localStorage(即时) + 后端DB(跨设备)

frontend/src/hooks/useLogin.tsx

@@ -1,6 +1,8 @@
 import { useState, useCallback } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { useAuth } from '@/hooks/useAuth';
+import { useTheme } from '@/hooks/useTheme';
+import api from '@/api/request';
 
 export interface LoginState {
   username: string;
@@ -15,6 +17,7 @@ export interface LoginState {
 export function useLogin(): LoginState {
   const navigate = useNavigate();
   const { login } = useAuth();
+  const { setTheme } = useTheme();
   const [username, setUsername] = useState('');
   const [password, setPassword] = useState('');
   const [loading, setLoading] = useState(false);
@@ -26,13 +29,19 @@ export function useLogin(): LoginState {
     setLoading(true);
     setError('');
     const ok = await login(username, password);
-    setLoading(false);
     if (ok) {
+      // Apply user preferences from backend (cross-browser sync)
+      try {
+        const prefs = await api.get<{ theme?: string; login_style?: string }>('/user/preferences');
+        if (prefs?.theme) setTheme(prefs.theme as 'dark' | 'light');
+        if (prefs?.login_style) localStorage.setItem('login-style', prefs.login_style);
+      } catch { /* ignore */ }
       navigate('/dashboard');
     } else {
       setError('用户名或密码错误');
     }
-  }, [username, password, login, navigate]);
+    setLoading(false);
+  }, [username, password, login, navigate, setTheme]);
 
   return { username, password, loading, error, setUsername, setPassword, handleLogin };
 }