feat(studio): add M (mute) and Shift+L (loop) keyboard shortcuts

[HOSS1E]

Summary

Adds NLE-style keyboard shortcuts for muting audio and toggling loop playback in the Studio player, matching the conventions in DaVinci Resolve, Premiere, and Final Cut.

• M — toggle audio mute
• ⇧L — toggle loop playback

Fixes #905.

Implementation notes

• Both handlers live in usePlaybackKeyboard.ts alongside the existing J / K / L / I / O / A / E shortcuts. The Shift+L case runs before the existing plain-L shuttle case so Shift+L doesn't also kick off forward playback.
• M mirrors the mute button's gating: above 1x playback, audio is force-muted by shouldMutePreviewAudio, so the shortcut becomes a no-op there (just like the button is disabled).
• The issue mentioned Ctrl+L as an alternative. It's intentionally not wired up because shouldIgnorePlaybackShortcutEvent already filters out events with ctrlKey/metaKey/altKey, and Ctrl/Cmd+L conflicts with the browser address-bar shortcut. Shift+L is also consistent with the existing Shift+I / Shift+O modifier pattern in this hook.
• The in-app shortcuts help panel (PlayerControls.tsx) lists the two new shortcuts in the "Playback" section.

Test plan

• bun run --filter @hyperframes/studio test — all 601 studio tests pass, including 4 new tests covering: M toggles mute, M is a no-op above 1x, Shift+L toggles loop without starting shuttle, plain L still starts shuttle (regression guard).
• bunx oxlint + bunx oxfmt --check on changed files — clean.
• bun run --filter @hyperframes/studio typecheck — clean.
• Manual verification: open Studio, focus the player, press M (mute icon toggles), press Shift+L (loop icon toggles), confirm plain L still starts forward shuttle.

[miguel-heygen]

Clean addition. Small scope, well-tested.

Verified:

• Shift+L handler placed before the existing plain L handler — correctly prevents fall-through to forward shuttle
• Both handlers placed after if (e.repeat) return — holding the key doesn't rapid-fire toggle
• M mute gating (playbackRate <= 1) matches the mute button's disabled prop — consistent behavior
• Ctrl+L intentionally omitted — shouldIgnorePlaybackShortcutEvent already filters ctrlKey/metaKey/altKey, and Ctrl+L conflicts with browser address bar
• 4 tests cover: toggle on/off, no-op above 1x, Shift+L isolation from shuttle, plain L regression guard
• Help panel updated with both shortcuts in the Playback section