Convert Astro config to TypeScript

Rename astro.config.mjs to astro.config.ts for full type inference
and better IDE support. Removes the // @ts-check workaround in favor
of native TypeScript.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kylemclaren

astro.config.mjs astro.config.tsastro.config.mjs renamed to astro.config.ts

@@ -1,5 +1,3 @@
-// @ts-check
-
 import react from '@astrojs/react';
 import sitemap from '@astrojs/sitemap';
 import starlight from '@astrojs/starlight';