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>

app/(marketing)/releases/components/mdx-components.tsx

@@ -6,10 +6,7 @@
 
 import Image from 'next/image'
 import Link from 'next/link'
-
-type MDXComponents = {
-  [key: string]: React.ComponentType<any>
-}
+import type { MDXComponents } from 'mdx/types'
 
 export const mdxComponents: MDXComponents = {
   // Headings with anchor links