fix: config.json lives in /app/data/ volume, not baked into image

- entrypoint copies example config to /app/data/config.json on first run
- symlinks /app/config.json → /app/data/config.json so app code unchanged
- theme.json also symlinked from /app/data/ if present
- config persists across container rebuilds without extra bind mounts
- updated README with new config/theme instructions

docker/entrypoint.sh

@@ -1,9 +1,17 @@
 #!/bin/sh
 
-# Copy example config if no config.json exists
-if [ ! -f /app/config.json ]; then
-  echo "[entrypoint] No config.json found, copying from config.example.json"
-  cp /app/config.example.json /app/config.json
+# Copy example config if no config.json exists (check volume mount first)
+if [ ! -f /app/data/config.json ]; then
+  echo "[entrypoint] No config.json found in /app/data/, copying example"
+  cp /app/config.example.json /app/data/config.json
+fi
+
+# Symlink so the app finds it at /app/config.json
+ln -sf /app/data/config.json /app/config.json
+
+# Same for theme.json (optional)
+if [ -f /app/data/theme.json ]; then
+  ln -sf /app/data/theme.json /app/theme.json
 fi
 
 exec /usr/bin/supervisord -c /etc/supervisor/conf.d/supervisord.conf