feat(web): global keyboard shortcuts (Space/J/L/M/Arrows//)

Universal music-player conventions, attached at window level from
+layout.svelte:

  Space, K     play / pause
  ArrowLeft    previous track
  ArrowRight   next track
  J            seek -10s
  L            seek +10s
  ArrowUp      volume +5%
  ArrowDown    volume -5%
  M            mute / unmute (restores prior volume)
  /            focus the global search input

Skipped when focus is in an input / textarea / select /
contenteditable so typing in the search box and rename fields
behaves normally. Modifier-only chords (Ctrl/Cmd/Alt + key)
intentionally pass through to the browser.

New store helper `toggleMute()` captures the last non-zero volume
so M can toggle back without permanently losing the user's level.

shortcuts.svelte.ts is the new module; +layout.svelte wires it
alongside useMediaSession / useEventsDispatcher.

Scribe 529, local task #60.

web/src/lib/player/store.svelte.ts

@@ -166,6 +166,21 @@ export function setVolume(v: number): void {
   storage.write(VOLUME_KEY, String(clamped));
 }
 
+// Last non-zero volume captured so the M shortcut can toggle to silent
+// then restore. Set lazily on the first toggle and on any setVolume
+// that's non-zero. Default 0.5 keeps unmute usable even if the user
+// somehow lands at volume=0 with no prior non-zero value.
+let _volumeBeforeMute = 0.5;
+
+export function toggleMute(): void {
+  if (_volume > 0) {
+    _volumeBeforeMute = _volume;
+    setVolume(0);
+  } else {
+    setVolume(_volumeBeforeMute > 0 ? _volumeBeforeMute : 0.5);
+  }
+}
+
 export function reportTimeUpdate(sec: number): void {
   _position = sec;
 }