feat: site-wide Swing Music player, persistent storage, mobile UX polish

- SiteWideMusicDock component at app level; persists across all routes
- Dock + player merged into single expandable panel
- Remove non-functional Hide button; one Minimize/Open control
- Swing Music label and action button on same row
- Mobile: dock z-index above player, larger touch target on iPad
- entrypoint.sh: seeds admin-content.json only on first boot/fresh volume
- Dockerfile: seed in /app/data-seed; live data in /app/data (volume)
- shell-with-music-bar padding on project and admin pages

.gitignore

@@ -12,6 +12,12 @@ dist
 dist-ssr
 *.local
 
+# Live admin data — persists on the server via the docker-compose bind mount.
+# The seed copy kept in data/admin-content.json is committed separately as
+# the image default and is preserved here for the Docker build context only.
+# Uncomment the line below if you do NOT want to track live data in git.
+# data/admin-content.json
+
 # Editor directories and files
 .vscode/*
 !.vscode/extensions.json