fix: correct start script to use dist/main.js instead of dist/index.js

The build:backend script outputs to dist/main.js, but the start script
was trying to run dist/index.js, causing MODULE_NOT_FOUND errors.

package.json

@@ -21,7 +21,7 @@
     "dev": "run-p 'dev:*'",
     "dev:frontend": "vite",
     "dev:backend": "NODE_ENV=development tsx watch src/server/main.ts --env-file-if-exists=.env.local",
-    "start": "node dist/index.js",
+    "start": "node dist/main.js",
     "build": "./scripts/build.sh",
     "build:frontend": "vite build",
     "build:backend": "esbuild src/server/main.ts --format=esm --bundle --packages=external --sourcemap --platform=node --outfile=dist/main.js",