feat: add frontend UI for SimpleNote Web

- Vanilla JS frontend with dark theme
- Dashboard with sidebar (libraries tree, tags), document grid, search
- Document viewer with markdown rendering and metadata panel
- Document editor with split write/preview and formatting toolbar
- Login screen with token authentication
- All styled according to UI/UX specs (dark theme, accent #00d4aa)
- API client for all endpoints
- Responsive design

package.json

@@ -18,6 +18,7 @@
     "express": "^4.21.0",
     "gray-matter": "^4.0.3",
     "js-yaml": "^4.1.0",
+    "marked": "^11.1.0",
     "uuid": "^9.0.1"
   },
   "devDependencies": {