feat: add image fetch with proxy support and registry proxy config

Backend:
- Add image fetch handler using go-containerregistry
- Support HTTP/HTTPS and SOCKS5 proxy protocols
- Pull images from remote registries (Docker Hub, etc.)
- Store fetched images as blobs and manifests
- Add timeout handling (60s for descriptor fetch)
- Add detailed logging for pull progress
- Add /api/v1/registry/image/fetch endpoint

Frontend:
- Add registry proxy configuration field
- Add "获取镜像" button and dialog
- Add proxy checkbox and input in fetch dialog
- Add LinearProgress feedback during fetch
- Add multi-stage Snackbar notifications
- Auto-refresh image list after successful fetch
- Fix download filename regex (remove trailing quote)
- Adjust button colors for better UI consistency

Dependencies:
- Add github.com/google/go-containerregistry for OCI operations
- Add golang.org/x/net/proxy for SOCKS5 support

🤖 Generated with [Qoder](https://qoder.com)

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