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

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

packages/studio/src/player/components/PlayerControls.tsx

@@ -15,6 +15,8 @@ const SHORTCUT_SECTIONS = [
       { key: "J", label: "Play backward" },
       { key: "K", label: "Stop" },
       { key: "L", label: "Play forward" },
+      { key: "M", label: "Toggle mute" },
+      { key: "⇧L", label: "Toggle loop" },
       { key: "←/→", label: "Step 1 frame" },
       { key: "⇧←/⇧→", label: "Step 10 frames" },
     ],

packages/studio/src/player/hooks/usePlaybackKeyboard.test.ts

@@ -172,3 +172,58 @@ describe("usePlaybackKeyboard — keyboard layout independence (#834)", () => {
     expect(spies.play).toHaveBeenCalledTimes(1);
   });
 });
+
+describe("usePlaybackKeyboard — mute & loop shortcuts (#905)", () => {
+  it("M toggles audioMuted", () => {
+    const { dispatch } = setupHook();
+    expect(usePlayerStore.getState().audioMuted).toBe(false);
+
+    act(() => {
+      dispatch(keydown({ code: "KeyM", key: "m" }));
+    });
+    expect(usePlayerStore.getState().audioMuted).toBe(true);
+
+    act(() => {
+      dispatch(keydown({ code: "KeyM", key: "m" }));
+    });
+    expect(usePlayerStore.getState().audioMuted).toBe(false);
+  });
+
+  it("M does NOT toggle audioMuted above 1x playback (matches button gating)", () => {
+    const { dispatch } = setupHook();
+    usePlayerStore.setState({ playbackRate: 2, audioMuted: false });
+
+    act(() => {
+      dispatch(keydown({ code: "KeyM", key: "m" }));
+    });
+
+    expect(usePlayerStore.getState().audioMuted).toBe(false);
+  });
+
+  it("Shift+L toggles loopEnabled without starting forward shuttle", () => {
+    const { dispatch, spies } = setupHook();
+    expect(usePlayerStore.getState().loopEnabled).toBe(false);
+
+    act(() => {
+      dispatch(keydown({ code: "KeyL", key: "L", shiftKey: true }));
+    });
+    expect(usePlayerStore.getState().loopEnabled).toBe(true);
+    expect(spies.play).not.toHaveBeenCalled();
+
+    act(() => {
+      dispatch(keydown({ code: "KeyL", key: "L", shiftKey: true }));
+    });
+    expect(usePlayerStore.getState().loopEnabled).toBe(false);
+  });
+
+  it("Plain L still starts forward shuttle (regression guard)", () => {
+    const { dispatch, spies } = setupHook();
+
+    act(() => {
+      dispatch(keydown({ code: "KeyL", key: "l" }));
+    });
+
+    expect(spies.play).toHaveBeenCalledTimes(1);
+    expect(usePlayerStore.getState().loopEnabled).toBe(false);
+  });
+});

packages/studio/src/player/hooks/usePlaybackKeyboard.ts

@@ -108,6 +108,21 @@ export function usePlaybackKeyboard({
         return;
       }
       if (e.repeat) return;
+      if (key === "m") {
+        e.preventDefault();
+        const state = usePlayerStore.getState();
+        // Audio is force-muted above 1x playback — match the mute button's gating.
+        if (state.playbackRate <= 1) {
+          state.setAudioMuted(!state.audioMuted);
+        }
+        return;
+      }
+      if (key === "l" && e.shiftKey) {
+        e.preventDefault();
+        const state = usePlayerStore.getState();
+        state.setLoopEnabled(!state.loopEnabled);
+        return;
+      }
       if (key === "k") {
         e.preventDefault();
         pause();