feat: play real music from local library in Memories slideshow

Scans MUSIC_DIR (mounted at /music in Docker) with music-metadata to
read embedded BPM/genre tags and classify tracks as upbeat/cinematic/
dreamy. Falls back to untagged general pool when mood bucket is thin.
Slideshow plays real audio via <audio> element with range-request
streaming; Web Audio synth is kept as fallback when MUSIC_DIR is unset.
Shows now-playing title/artist in the slideshow bottom bar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -19,6 +19,7 @@
     "exifr": "^7.1.3",
     "express": "^4.19.2",
     "leaflet": "^1.9.4",
+    "music-metadata": "^11.15.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-leaflet": "^4.2.1",