Fix Vite proxy inside Docker and add success pages

- vite.config.ts: proxy target via VITE_API_TARGET env var (falls back to localhost)
- docker-compose.dev.yml: set VITE_API_TARGET=http://backend:8000
- Add /login-success and /register-success placeholder pages
- Show real API error messages in login/register forms

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

docker-compose.dev.yml

@@ -15,6 +15,8 @@ services:
     command: npm run dev -- --host 0.0.0.0
     ports:
       - "5173:5173"
+    environment:
+      VITE_API_TARGET: http://backend:8000
     volumes:
       - ./frontend:/app
       - /app/node_modules