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/components/theme-toggle.tsx

@@ -0,0 +1,26 @@
+"use client";
+
+import { Moon, Sun } from "lucide-react";
+import { useTheme } from "@/lib/use-theme";
+
+export function ThemeToggle({ className }: { className?: string }) {
+  const { theme, toggle, mounted } = useTheme();
+
+  return (
+    <button
+      type="button"
+      onClick={toggle}
+      aria-label={mounted ? (theme === "dark" ? "切换到浅色" : "切换到深色") : "切换主题"}
+      className={
+        className ??
+        "flex h-full w-full items-center justify-center text-foreground/80 hover:text-foreground"
+      }
+    >
+      {mounted && theme === "dark" ? (
+        <Sun className="h-5 w-5" />
+      ) : (
+        <Moon className="h-5 w-5" />
+      )}
+    </button>
+  );
+}