Fix TOC button to be truly fixed-position using React Portal

The TOC toggle button was appearing near the end of posts instead of
floating at a fixed position. This happened because the button was
rendered inside the PostLayout component hierarchy.

Changes:
- Use React Portal to render TOC button at document.body level
- Add mounted state for proper SSR/client hydration
- Button now floats like back-to-top button, visible from start

This ensures the button is always visible and accessible, similar to
the back-to-top button behavior.

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

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

next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/types/routes.d.ts";
+import "./.next/dev/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.