frontend: rebuild bookmark page with drag-and-drop, search, and theme system

- bookmark management with dnd-kit reordering, bulk edit, search,
  category filter/rename, and meta auto-fetch
- migrate /bookmarks → /dashboard/bookmarks under (main) layout
- homepage redesign with category grid, /-key search, dock tooltips
- theme toggle + use-theme, sonner toasts, alert-dialog/skeleton,
  visual refresh of auth pages

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/app/init/page.tsx

@@ -64,7 +64,7 @@ export default function InitPage() {
 
   if (loading) {
     return (
-      <div className="flex min-h-screen items-center justify-center">
+      <div className="flex min-h-screen items-center justify-center bg-gradient-to-br from-background via-background to-muted/40 text-sm text-muted-foreground">
         加载中...
       </div>
     );
@@ -72,13 +72,13 @@ export default function InitPage() {
 
   if (initialized) {
     return (
-      <div className="flex min-h-screen items-center justify-center p-4">
+      <div className="flex min-h-screen items-center justify-center bg-gradient-to-br from-background via-background to-muted/40 p-4">
         <Card className="w-full max-w-md">
           <CardHeader>
             <CardTitle className="text-center">系统已初始化</CardTitle>
           </CardHeader>
           <CardContent>
-            <p className="text-center text-gray-500">
+            <p className="text-center text-sm text-muted-foreground">
               系统中已有用户，无法再次初始化。
             </p>
           </CardContent>
@@ -88,13 +88,13 @@ export default function InitPage() {
   }
 
   return (
-    <div className="flex min-h-screen items-center justify-center bg-gray-50 p-4">
+    <div className="flex min-h-screen items-center justify-center bg-gradient-to-br from-background via-background to-muted/40 p-4">
       <Card className="w-full max-w-md">
         <CardHeader>
           <CardTitle className="text-center">系统初始化</CardTitle>
         </CardHeader>
         <CardContent className="space-y-4">
-          <p className="text-sm text-gray-500">
+          <p className="text-sm text-muted-foreground">
             这是系统首次启动，请创建第一个管理员账号。
           </p>
           <form onSubmit={handleSubmit} className="space-y-4">
@@ -139,7 +139,7 @@ export default function InitPage() {
                 required
               />
             </div>
-            {error && <p className="text-sm text-red-500">{error}</p>}
+            {error && <p className="text-sm text-destructive">{error}</p>}
             <Button type="submit" className="w-full">
               创建管理员
             </Button>