fix: update legacy runtime path

- Changed the legacy runtime path from 'legacy-runtime.js' to 'index.js'
in runtime configuration.
- Added a new start script (start.js) to check for the existence of the
built TypeScript server entrypoint and import it.

server/start.js

@@ -0,0 +1,18 @@
+#!/usr/bin/env node
+import fs from 'fs';
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const distEntrypoint = path.join(__dirname, 'dist', 'bootstrap.js');
+
+if (!fs.existsSync(distEntrypoint)) {
+  console.error(
+    '[server] Missing built TypeScript server entrypoint at server/dist/bootstrap.js. ' +
+    'Run "npm run server" for development or "npm run server:build" before "npm run server:start".'
+  );
+  process.exit(1);
+}
+
+await import('./dist/bootstrap.js');