Splitscreen: Add toggle for shared vs per-panel lyrics display

[carochacs]

Description

When using splitscreen mode with lyrics enabled, lyrics should be toggleable between two display modes:

1. Shared (single display) — lyrics displayed once, centered or in a dedicated panel
2. Per-panel — lyrics displayed in each splitscreen panel independently

Use Case

When practicing with multiple arrangement views (e.g., lead and rhythm guitar simultaneously), users should be able to:

• Display lyrics once (shared) to avoid redundancy and clutter
• OR display lyrics in each panel independently to see which arrangement each lyric line corresponds to

Implementation Considerations

• Add a toggle in splitscreen UI or lyrics plugin settings
• Persist preference in localStorage
• Detect splitscreen mode and adapt lyrics display accordingly
• Works with both lyrics-sync and lyrics-karaoke plugins
• Consider panel-scoped shortcuts for toggling

Related

• Splitscreen architecture: each panel has independent highway instance
• Lyrics plugins: lyrics-sync, lyrics-karaoke
• WebSocket stream: window.feedBack event emitter