fix: improve iOS download handoff and subscription feedback

[Priveetee]

Summary

• add an iOS WebKit downloader handoff that waits for job completion, then asks for one final tap to open the native iOS share sheet with the generated artifact file.
• keep non-iOS behavior unchanged by preserving automatic artifact open on other mobile/desktop paths.
• harden subscribe/unsubscribe UX in watch and shorts overlays by awaiting mutations and surfacing failure feedback instead of optimistic success toasts.

Included Commits

• 9d09dce fix: surface subscription failures in watch and shorts
• af0af78 feat: add iOS share-sheet handoff for downloader artifacts

Validation

• bun run check
• bun run sherif
• bun run knip
• bun run build