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>

astro.config.mjs

@@ -5,14 +5,13 @@ import sitemap from "@astrojs/sitemap";
 
 import react from "@astrojs/react";
 import tailwindcss from "@tailwindcss/vite";
-import netlify from "@astrojs/netlify";
+// Admin API runs as a standalone Netlify Function (see netlify/functions/admin.mts)
 
 // https://astro.build/config
 export default defineConfig({
   site: "https://interactives.neeldhara.com",
   integrations: [mdx(), sitemap(), react()],
   output: "static",
-  adapter: netlify(),
 
   fonts: [
     {