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>

lib/mdx/releases.ts

@@ -7,7 +7,6 @@
 import fs from 'fs'
 import path from 'path'
 import matter from 'gray-matter'
-import { MDXRemote } from 'next-mdx-remote/rsc'
 
 const RELEASES_DIR = path.join(process.cwd(), 'content/nl/releases')