fix: skip unavailable Spotify tracks during playback

[gjermundgaraba]

I noticed when we hit unavailable tracks on Spotify, the player would just derp out and stop.
Fixing this was surprisingly gnarly, but I think the approach taken here is quite robust and can be reused for other providers if they have similar concepts.

Design decisions:

• Spotify tracks that are marked unplayable are kept in the playlist and flagged on the shared playlist.Track struct instead of being dropped from playlist navigation. This keeps the visible playlist aligned with Spotify, avoids index mismatches, and makes the flag available to any provider.
• Skip logic now lives in the playlist package. Next(), Prev(), PeekNext(), and ActivateSelected() resolve playable tracks internally, so the UI no longer needs retry loops or provider-specific playability checks.
• Navigation uses rollback-on-failure for playlist position/current track: if no playable track is found, the active position is restored and false is returned, so the UI cursor and playback state do not drift. Unplayable queued entries are still pruned when consumed.
• Unplayable queue entries are pruned lazily when advancing playback rather than eagerly removed, which avoids extra change propagation.
• ActivateSelected() separates selection from activation: it resolves the selected row to the next playable track in playlist order, reports whether a skip occurred, and lets the UI show a message without owning playability rules.
• Unplayable tracks are rendered in the playlist as dimmed rows with an (unavailable) suffix, so skipped items remain visible to the user.