FileBrowser: diagnose container mounts + support extra audio path

This is a Docker volume issue, not a code bug. The backend container
only sees paths explicitly mounted in docker-compose.yml. A new mount
on the Linux host (e.g. NAS share, USB drive) is invisible to the
container until added as a volume — restarting the container alone
doesn't help, and restarting just the host doesn't either.

Backend: New GET /api/filebrowser/diagnose endpoint reads
/proc/self/mountinfo and returns the actual bind/nfs/cifs mounts the
container sees, plus a check of common candidate roots (/audiofiles,
/mnt, /media, /srv, /home, /app/data) showing whether they exist and
have content.

Frontend: Info icon in FileBrowser header toggles a diagnose panel
showing mounts and root candidates. Quick-access buttons now built
dynamically from candidate roots that actually exist. On 'path not
found' error: helpful inline explanation including the exact .env
variable and docker-compose command needed to add a new mount.

docker-compose.yml: New EXTRA_AUDIO_PATH env variable. Mounts a
second host path 1:1 (same path inside container, like
AUDIOFILES_PATH does). Defaults to ./data → /extra_audio_unused
when unset, which is a no-op.

.env.example: Documents EXTRA_AUDIO_PATH usage with example.

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

docker-compose.yml

@@ -7,6 +7,9 @@ services:
     volumes:
       - ./data:/app/data
       - ${AUDIOFILES_PATH:-./audiofiles}:${AUDIOFILES_PATH:-/audiofiles}:ro
+      # Zusätzlicher Pfad (NAS-Share, USB-Platte, weiterer Mount):
+      # Setze EXTRA_AUDIO_PATH=/host/pfad in .env. Default ist ein No-Op.
+      - ${EXTRA_AUDIO_PATH:-./data}:${EXTRA_AUDIO_PATH:-/extra_audio_unused}:ro
     expose:
       - "8000"
     depends_on: []