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>

pnpm-lock.yaml

@@ -75,6 +75,9 @@ importers:
       '@tailwindcss/postcss':
         specifier: ^4
         version: 4.1.17
+      '@types/mdx':
+        specifier: ^2.0.13
+        version: 2.0.13
       '@types/node':
         specifier: ^20
         version: 20.19.24