Add multiple playlist support; fix settings save in container

- PLAYLIST_NAMES config key (comma-separated list); falls back to PLAYLIST_NAME
- run_update_job syncs songs to every configured playlist, merges results
- Settings form: dynamic add/remove playlist rows instead of single input
- get_web_config reads config.json directly so form is consistent in
  container environments where env vars override runtime config
- Playlist tab: dropdown to pick which playlist to browse when multiple exist

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

journeyfm/config_store.py

@@ -23,6 +23,7 @@ DEFAULT_CONFIG = {
 NON_SECRET_KEYS = {
     "SERVER_IP",
     "PLAYLIST_NAME",
+    "PLAYLIST_NAMES",
     "AUTO_UPDATE",
     "UPDATE_INTERVAL",
     "UPDATE_UNIT",