Add per-node scene music volume control

Author a music track and volume (0-100%) per story node; the runtime
scales it to 0-1 and applies it without restarting the track when only
the level changes. Adds migration 024, threads music_volume through the
repository/runtime, and gives the node inspector a volume slider.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/App.tsx

@@ -155,7 +155,7 @@ export function App() {
 
   // Scene music follows the current node (null inherits; a finished playthrough stops).
   useEffect(() => {
-    if (runtimeNode?.musicUrl) audio.setMusic(runtimeNode.musicUrl)
+    if (runtimeNode?.musicUrl) audio.setMusic(runtimeNode.musicUrl, runtimeNode.musicVolume)
     else if (playthrough?.status === 'finished') audio.setMusic(null)
   }, [runtimeNode, playthrough])