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/package.json

@@ -10,6 +10,9 @@
   },
   "dependencies": {
     "@base-ui/react": "^1.4.0",
+    "@dnd-kit/core": "^6.3.1",
+    "@dnd-kit/sortable": "^10.0.0",
+    "@dnd-kit/utilities": "^3.2.2",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "framer-motion": "^12.38.0",
@@ -21,6 +24,7 @@
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "shadcn": "^4.2.0",
+    "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",
     "tw-animate-css": "^1.4.0"
   },