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/routes/+layout.svelte

@@ -20,6 +20,7 @@
   } from '$lib/player/store.svelte';
   import { useMediaSession } from '$lib/player/mediaSession.svelte';
   import { useEventsDispatcher } from '$lib/player/events.svelte';
+  import { useGlobalShortcuts } from '$lib/player/shortcuts.svelte';
   import { applyMetaThemeColor } from '$lib/theme/applyMetaThemeColor.svelte';
   import { audioLoader } from '$lib/player/audioLoader';
   import { attachStallRetry } from '$lib/player/stallRetry';
@@ -113,6 +114,7 @@
 
   useMediaSession();
   useEventsDispatcher();
+  useGlobalShortcuts();
   applyMetaThemeColor();
 </script>