Move dev servers to ports 8002 and 5172

Backend now runs on 8002 and the frontend dev server on 5172, with the
proxy, CORS origins and the run_*.bat launchers updated to match. The
launchers also activate the venv so they work from a clean shell.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/.env.example

@@ -1 +1,2 @@
-REACT_APP_API_URL=http://localhost:8000
+PORT=5172
+REACT_APP_API_URL=http://localhost:8002

frontend/package.json

@@ -12,6 +12,7 @@
   },
   "scripts": {
     "start": "react-scripts start",
+    "dev": "react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test",
     "eject": "react-scripts eject"
@@ -33,5 +34,5 @@
       "last 1 safari version"
     ]
   },
-  "proxy": "http://localhost:8000"
+  "proxy": "http://localhost:8002"
 }