- Global error handler: friendly messages for 429, 502, 403, 400
instead of raw tracebacks. Filter FetchError from Flask logger.
- Fix None URLs in templates: protect href/src in common_elements,
playlist, watch, and comments templates against None values.
- Radio playlists (RD...): redirect /playlist?list=RD... to
/watch?v=...&list=RD... since YouTube only supports them in player.
- Wrap player client fallbacks (ios, tv_embedded) in try/catch so
a failed fallback doesn't crash the whole page.