fix: use Node.js wrapper for cross-platform PORT fallback

Replace cross-env with a simple Node.js wrapper script that:
- Respects externally supplied PORT environment variable
- Falls back to 8502 if PORT is not set
- Works cross-platform without extra dependencies
- No runtime dependencies beyond Node.js itself

Changes:
- Add start-server.js wrapper script
- Update package.json to use wrapper
- Remove cross-env dependency
- Copy start-server.js in Dockerfile

This fixes both issues:
1. Preserves PORT fallback behavior (PORT can be overridden)
2. No extra runtime dependencies needed

frontend/package.json

@@ -5,7 +5,7 @@
   "scripts": {
     "dev": "next dev",
     "build": "next build",
-    "start": "cross-env PORT=8502 node .next/standalone/server.js",
+    "start": "node start-server.js",
     "lint": "next lint"
   },
   "dependencies": {
@@ -55,7 +55,6 @@
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",
-    "cross-env": "^10.1.0",
     "eslint": "^9",
     "eslint-config-next": "15.4.2",
     "tailwindcss": "^4",