Switch to standalone Netlify Function for admin API

Remove Astro adapter (causes esbuild bundling errors on Netlify)
and use a pure static build + standalone Netlify Function instead.

- Removed @astrojs/netlify adapter — site builds as fully static
- Created netlify/functions/admin.mts as a standalone Netlify Function
- Function handles all admin API operations via GitHub API
- Removed src/pages/api/admin.ts (Astro SSR route)
- Local dev: admin syncs to localStorage (server sync fails silently)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

netlify.toml

@@ -1,6 +1,7 @@
 [build]
   command = "npm run build"
   publish = "dist"
+  functions = "netlify/functions"
 
 [build.environment]
   NODE_VERSION = "22"