Replace * key new-releases mode with inline background sync

[Jason-Abbott]

• Export SyncArtist from check package
• Remove newReleasesMode from TUI (mode, modal, filter, help)
• Add sync infrastructure (SyncArtistFunc, fields, messages)
• Implement * key handler with toggle (start/cancel)
• Handle syncArtistDoneMsg in Update
• Update spinner tick, renderItem, help text
• Inject SyncArtistFunc from cmd/root.go
• Build, test, lint

Summary of Changes

Replaced the * key new-releases mode with inline background sync. Pressing * now syncs all followed artists from MusicBrainz (skipping recently-checked). The checkmark next to each artist becomes a spinner while syncing. Pressing * again cancels. Removed the newReleasesMode filtered view, the new releases modal, and the unused openFile/searchWeb functions.

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