Add a configurable API timeout for Jellyfin bulk requests and improve fetch retry behavior. UI: adds an "API Timeout (seconds)" field (15–300s, default 30) in webui/index.html and persists it via webui/static/script.js (load/save). Client: jellyfin_client.py now reads api_timeout from config_manager, uses it as the bulk timeout and computes a sensible non-bulk timeout (max(5, bulk_timeout//6)). Fetch loops for tracks and albums were hardened: reducing batch size now resets consecutive failure counters, log messages were clarified, and the stopping/retry thresholds were adjusted to avoid premature aborts at minimum batch sizes.
Introduces backend and frontend functionality to list and select Jellyfin music libraries. Adds API endpoints, updates the client logic, and provides a UI selector for users to choose their preferred Jellyfin music library.
