fix: Remove hardcoded ports, support custom port configuration

- Replace all hardcoded localhost:8000/3000/8501 with environment variables
- Frontend: Use API_PORT env var in next.config.ts
- Backend: Use API_PORT env var in editor.py and quality.py
- Web UI: Use API_PORT and EDITOR_PORT env vars in all Streamlit pages
- Update dev.sh to pass environment variables to all services
- Add .env.example with port configuration template

Now supports custom ports via environment variables:
  API_PORT=8080 EDITOR_PORT=3001 WEB_PORT=8502 ./dev.sh

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/next.config.ts

@@ -2,10 +2,11 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   async rewrites() {
+    const apiPort = process.env.API_PORT || '8000';
     return [
       {
         source: '/api/:path*',
-        destination: 'http://localhost:8000/api/:path*',
+        destination: `http://localhost:${apiPort}/api/:path*`,
       },
     ]
   },