Fix MUSIC_PATH direct mount + backend URL prefill

- MUSIC_PATH is now used 1:1 inside the container (no /music remapping)
- config.js 404-safe with inline fallback defaults
- Prefill uses .trim() so empty strings don't overwrite fields

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

docker-compose.yml

@@ -19,12 +19,12 @@ services:
     working_dir: /app
     volumes:
       - ./backend/novasonic-server.js:/app/novasonic-server.js:ro
-      - ${MUSIC_PATH:-/music}:/music   # your Navidrome music library
+      - ${MUSIC_PATH}:${MUSIC_PATH}      # exact path from .env, no remapping
     ports:
-      - "3700:3700"                    # route this port through your reverse proxy
+      - "3700:3700"                      # route this port through your reverse proxy
     environment:
       - PORT=3700
-      - MUSIC_PATH=/music
+      - MUSIC_PATH=${MUSIC_PATH}         # same path inside the container
       - AUDIO_FMT=${AUDIO_FMT:-mp3}
       - AUDIO_QUALITY=${AUDIO_QUALITY:-0}
       - MAX_PARALLEL=${MAX_PARALLEL:-2}