fix: resolve MDX type imports for release notes

Fixed runtime error in release notes pages:
- Removed unused MDXRemote import from lib/mdx/releases.ts
- Fixed MDXComponents type import in mdx-components.tsx
- Added @types/mdx as dev dependency

Pages now load correctly:
- /releases (overview)
- /releases/authentication
- /releases/release-notes-system

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

package.json

@@ -35,6 +35,7 @@
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",
+    "@types/mdx": "^2.0.13",
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",