[Task]: Shareable Gradient Links

### Summary

Add ability to copy/share a link encoding gradient state (JSON → base64).

### Milestone

1.5.0 — Share & Sync

### Details

_No response_

### Checklist

- [ ] Serialise {type, stops, angle} into compact format
- [ ] Generate shareable URL
- [ ] Add “Copy link” action to preview


### Priority

Low