feat(yunara): add Song detail page

[crrow]

Problem

There's no dedicated page to view song details. Clicking on a song just plays it.

Solution

Create a SongView pane item that displays:

• Song title, artist, album
• Cover art
• Duration
• Play button
• Add to playlist, share, etc. (placeholder buttons)

Navigation: clicking song title/info link from track lists opens SongView.

Files to modify

• Create: `yunara-player/src/pane/items/song_view.rs`
• Modify: `yunara-player/src/pane/items/mod.rs` — register SongView
• Modify: `yunara-player/src/actions.rs` — add Song variant to NavigateAction
• Modify: `yunara-player/src/yunara_player.rs` — handle Song navigation

Verification

• Song detail page renders with correct info
• Navigation from track lists works
• Back button returns to previous view

[crrow]

In progress — working in worktree .worktrees/issue-219 on branch feat/219-song-detail