fix: use cross-env for cross-platform PORT default

Replace POSIX-only PORT=${PORT:-8502} syntax with cross-env
to ensure npm run start works on Windows and other platforms.

- Install cross-env as dev dependency
- Update start script to use cross-env PORT=8502

frontend/package-lock.json

@@ -54,6 +54,7 @@
         "@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",
@@ -125,6 +126,13 @@
         "tslib": "^2.4.0"
       }
     },
+    "node_modules/@epic-web/invariant": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/@epic-web/invariant/-/invariant-1.0.0.tgz",
+      "integrity": "sha512-lrTPqgvfFQtR/eY/qkIzp98OGdNJu0m5ji3q/nJI8v3SXkRKEnWiOxMmbvcSoAIzv/cGiuvRy57k4suKQSAdwA==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@eslint-community/eslint-utils": {
       "version": "4.7.0",
       "resolved": "https://registry.npmjs.org/@eslint-community/eslint-utils/-/eslint-utils-4.7.0.tgz",
@@ -4038,6 +4046,24 @@
       "integrity": "sha512-/Srv4dswyQNBfohGpz9o6Yb3Gz3SrUDqBH5rTuhGR7ahtlbYKnVxw2bCFMRljaA7EXHaXZ8wsHdodFvbkhKmqg==",
       "dev": true
     },
+    "node_modules/cross-env": {
+      "version": "10.1.0",
+      "resolved": "https://registry.npmjs.org/cross-env/-/cross-env-10.1.0.tgz",
+      "integrity": "sha512-GsYosgnACZTADcmEyJctkJIoqAhHjttw7RsFrVoJNXbsWWqaq6Ym+7kZjq6mS45O0jij6vtiReppKQEtqWy6Dw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@epic-web/invariant": "^1.0.0",
+        "cross-spawn": "^7.0.6"
+      },
+      "bin": {
+        "cross-env": "dist/bin/cross-env.js",
+        "cross-env-shell": "dist/bin/cross-env-shell.js"
+      },
+      "engines": {
+        "node": ">=20"
+      }
+    },
     "node_modules/cross-spawn": {
       "version": "7.0.6",
       "resolved": "https://registry.npmjs.org/cross-spawn/-/cross-spawn-7.0.6.tgz",

frontend/package.json

@@ -5,7 +5,7 @@
   "scripts": {
     "dev": "next dev",
     "build": "next build",
-    "start": "PORT=${PORT:-8502} node .next/standalone/server.js",
+    "start": "cross-env PORT=8502 node .next/standalone/server.js",
     "lint": "next lint"
   },
   "dependencies": {
@@ -55,6 +55,7 @@
     "@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",