fix(docker-setup): avoid set -e trap in FileBrowser mount discovery

((found_mounts++)) with found_mounts=0 evaluates the post-increment
expression to 0, which bash treats as exit code 1 under set -e,
silently killing the script on the first mount discovered.

Use arithmetic assignment instead to avoid the zero-exit trap.

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

scripts/docker-setup.sh

@@ -1288,7 +1288,7 @@ install_filebrowser() {
             [[ "$name" == .felhom-raw ]] && continue
             volume_lines+="      - \"${mp%/}:/srv/${name}\""$'\n'
             mount_comment+="      # ${mp%/} → /srv/${name}"$'\n'
-            ((found_mounts++))
+            found_mounts=$(( found_mounts + 1 ))
         done
     fi