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

entrypoint.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+set -e
+
+DATA_FILE=/app/data/admin-content.json
+SEED_FILE=/app/data-seed/admin-content.json
+
+# Ensure the data directory exists (in case the volume was not mounted)
+mkdir -p /app/data
+
+# Only seed from image defaults when no live data file exists yet.
+# This runs on first boot or a fresh volume, but never overwrites existing data.
+if [ ! -f "$DATA_FILE" ] && [ -f "$SEED_FILE" ]; then
+  echo "[siteforge] No admin-content.json found. Seeding from image defaults..."
+  cp "$SEED_FILE" "$DATA_FILE"
+fi
+
+exec node server.js