Adds NLE-style hotkeys for muting audio and toggling loop playback in the
Studio player, matching the workflow conventions in DaVinci Resolve,
Premiere, and Final Cut.
- M toggles audio mute (no-op above 1x playback, matching the mute button's
existing gating behavior).
- Shift+L toggles loop. Ctrl/Cmd+L was considered but is filtered out by
shouldIgnorePlaybackShortcutEvent and conflicts with the browser address
bar; Shift+L is also consistent with the existing Shift+I / Shift+O
modifier pattern.
The Shift+L handler runs before the existing plain-L shuttle case so it
doesn't also start forward playback.
Fixes#905
