feat(studio): add playback hotkeys

[AjTheSpidey]

Adds the playback hotkeys from #905.

M toggles preview mute, and Shift+L / Ctrl+L toggle loop playback. Shift+L is handled before the normal JKL shuttle path so it does not accidentally start forward playback. I also added the new shortcuts to the keyboard panel in the player controls.

Checked locally:

• bun run --filter @hyperframes/studio typecheck
• bun run --filter @hyperframes/studio test -- src/player/hooks/usePlaybackKeyboard.test.ts
• bun run --filter @hyperframes/studio test -- src/player/hooks/useTimelinePlayer.test.ts
• bun run lint
• bun run format:check
• bun run --filter @hyperframes/studio build

Small Windows note: I had to run install with a local Bun cache and --ignore-scripts because the prepare hook calls Unix test here, but the checks above are the ones I used before opening this.

Closes #905