feat: add docker-compose.dev.yml for local development

- docker-compose.dev.yml: backend on 8000, frontend (Vite) on 5173
- Backend mounts source files + uvicorn --reload for hot reload
- Frontend uses node:20-alpine, mounts ./frontend, runs npm run dev --host
- vite.config.ts: proxy target reads from API_TARGET env var
  (defaults to localhost:8000 for plain npm run dev,
   set to http://backend:8000 by docker-compose.dev.yml)

Usage: docker compose -f docker-compose.dev.yml up

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flight-comparator/frontend/vite.config.ts

@@ -1,20 +1,18 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 
+// When running inside Docker the backend is reachable via its service name.
+// Outside Docker (plain `npm run dev`) it falls back to localhost.
+const apiTarget = process.env.API_TARGET ?? 'http://localhost:8000'
+
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [react()],
   server: {
     port: 5173,
     proxy: {
-      '/api': {
-        target: 'http://localhost:8000',
-        changeOrigin: true,
-      },
-      '/health': {
-        target: 'http://localhost:8000',
-        changeOrigin: true,
-      }
+      '/api': { target: apiTarget, changeOrigin: true },
+      '/health': { target: apiTarget, changeOrigin: true },
     }
   }
 })