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/login-dialog.tsx

@@ -21,7 +21,7 @@ export function LoginDialog({
 
   return (
     <Dialog open={open} onOpenChange={setOpen}>
-      <DialogTrigger className="rounded-lg bg-slate-900 px-5 py-2.5 text-sm font-medium text-white hover:bg-slate-800">
+      <DialogTrigger className="rounded-lg bg-primary px-5 py-2.5 text-sm font-medium text-primary-foreground transition-colors hover:bg-primary/90 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2">
         {children ?? "登录"}
       </DialogTrigger>
       <DialogContent className="sm:max-w-md">