feat: add aturi.to as share option
Add aturi.to universal link sharing to post share menu, allowing users
to share content that recipients can view on any ATProto client.
Changes:
- Add toShareUrlAturi() function to convert witchsky URLs to aturi.to format
- Implement onSharePostAturi() and onCopyLinkAturi() handlers with analytics
- Add "Share via aturi.to..." and "Copy aturi.to link" menu items
aturi.to links let recipients choose their preferred client (Bluesky,
Blacksky, Anisota, etc.) instead of forcing them to a specific platform.