🔗 Shareable Repository Tree Links

### Goals
Allow users to generate a shareable URL that preserves:
- repository source (GitHub/GitLab)
- selected options (view mode, settings)
- generated tree

### Scope
- [ ] Encode repo + options into URL params
- [ ] Auto-load tree from URL
- [ ] Add "Copy Share Link" button
- [ ] Ensure links are stable and reproducible