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/pages/LoginPage.tsx

@@ -1,3 +1,4 @@
+import { useState, useEffect } from 'react';
 import SplitView from '@/components/login/SplitView';
 import CenteredCard from '@/components/login/CenteredCard';
 import SidePanel from '@/components/login/SidePanel';
@@ -22,15 +23,32 @@ export const LOGIN_STYLE_OPTIONS: { id: LoginStyle; name: string; desc: string }
   { id: 'minimal', name: '极简暗色', desc: '极致简约，纯暗色风格' },
 ];
 
-function getLoginStyle(): LoginStyle {
-  if (typeof window === 'undefined') return 'split';
-  const stored = localStorage.getItem('login-style') as LoginStyle | null;
-  if (stored && stored in LOGIN_STYLES) return stored;
-  return 'split';
-}
-
 export default function LoginPage() {
-  const style = getLoginStyle();
+  const [style, setStyle] = useState<LoginStyle>('split');
+  const [ready, setReady] = useState(false);
+
+  useEffect(() => {
+    // Try backend first (cross-browser), fallback to localStorage
+    fetch('/api/v1/public/login-style')
+      .then(r => r.json())
+      .then(data => {
+        const s = data?.login_style as LoginStyle;
+        if (s && s in LOGIN_STYLES) {
+          setStyle(s);
+        } else {
+          const stored = localStorage.getItem('login-style') as LoginStyle | null;
+          if (stored && stored in LOGIN_STYLES) setStyle(stored);
+        }
+      })
+      .catch(() => {
+        const stored = localStorage.getItem('login-style') as LoginStyle | null;
+        if (stored && stored in LOGIN_STYLES) setStyle(stored);
+      })
+      .finally(() => setReady(true));
+  }, []);
+
+  if (!ready) return null; // avoid flash
+
   const Component = LOGIN_STYLES[style] || SplitView;
   return <Component />;
 }