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>

.env.example

@@ -4,8 +4,9 @@
 NAVIDROME_URL=https://music.yourdomain.com
 BACKEND_URL=https://novasonic-api.yourdomain.com
 
-# Path to your Navidrome music library on the host
-MUSIC_PATH=/path/to/your/navidrome/music
+# Absolute path to your Navidrome music library on the host
+# This exact path is used inside the container — no subdirectory is added
+MUSIC_PATH=/srv/navidrome/music
 
 AUDIO_FMT=mp3          # mp3 | flac | opus | best
 AUDIO_QUALITY=0        # 0 = best quality