feat: quick screenshot upload at /upload

- Drop zone: drag and drop files
- Paste: Ctrl+V pastes clipboard screenshots directly
- Browse: file picker button
- Saves to platform/screenshots/ with timestamp filename
- Mounted as volume in frontend container
- Accessible from any device at dash.quadjourney.com/upload

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -5,6 +5,8 @@ services:
       dockerfile: Dockerfile
     container_name: platform-frontend-v2
     restart: unless-stopped
+    volumes:
+      - ./screenshots:/app/screenshots
     environment:
       - ORIGIN=${PLATFORM_V2_ORIGIN:-https://dash.quadjourney.com}
       - GATEWAY_URL=http://gateway:8100