feat: Gitea Actions CI/CD, Docker registry images, drive drag-and-drop

- Add .gitea/workflows/docker-build.yml: builds backend and frontend
  Docker images on every push, pushes to Gitea Container Registry
  (git.necloud.us/nate/cloudlink/{backend,frontend}:latest + SHA tag)
- Add desktop build job: Linux AppImage and Windows NSIS installer via
  Wine cross-compilation, uploaded as workflow artifacts
- Add release job: creates a Gitea release with installers attached when
  a v* tag is pushed
- Add deploy job: SSH pull + restart on pushes to master
- docker-compose.yml: add image: fields alongside build: so the server
  can pull pre-built images with docker compose pull
- DrivePage: add drag-and-drop upload with full-screen drop overlay;
  refactor upload logic into a shared uploadFile callback

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

docker-compose.yml

@@ -20,6 +20,7 @@ services:
       retries: 5
 
   backend:
+    image: git.necloud.us/nate/cloudlink/backend:latest
     build:
       context: ./backend
       dockerfile: Dockerfile
@@ -39,6 +40,7 @@ services:
         condition: service_healthy
 
   frontend:
+    image: git.necloud.us/nate/cloudlink/frontend:latest
     build:
       context: ./frontend
       dockerfile: Dockerfile