chore: scaffold astro frontend with static output

frontend/astro.config.mjs

@@ -0,0 +1,19 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+import node from '@astrojs/node';
+import react from '@astrojs/react';
+
+// astro 5 merged 'hybrid' into 'static' + per-route `export const prerender = false`.
+// the node adapter is required so api endpoints can actually run on demand.
+export default defineConfig({
+  output: 'static',
+  adapter: node({ mode: 'standalone' }),
+  integrations: [react()],
+  server: {
+    host: process.env.HOST || '0.0.0.0',
+    port: Number(process.env.PORT) || 3000,
+  },
+  vite: {
+    server: { fs: { strict: true } },
+  },
+});