refactor: remove all playlist support, fix message ordering

- Remove playlist handler, keyboard, and all related callback routing
- Strip playlist parameter from YouTube Music radio URLs, reject pure playlists
- Send media preview before format selection prompt (correct ordering)
- Remove DetectPlaylist/GetPlaylistInfo methods (no longer needed)
- Remove IsPlaylist, PlaylistCount, PlaylistEntries from MediaInfo
- Remove PlaylistState struct, keep PlaylistEntry for search results
- Update README to reflect removal of playlist feature

README.md

@@ -10,7 +10,6 @@ Telegram bot for downloading media from any site yt-dlp supports.
 - Thumbnail preview with metadata (title, uploader, duration) before download
 - Audio-only sites (Spotify, SoundCloud, YouTube Music, etc.) auto-detect and skip to audio quality selection
 - Multi-step format selection: quality, secondary format (audio/video combine), language
-- Playlist support with paginated entry selection (10 per page, select all, confirm with count)
 - Download progress updates (with speed and ETA)
 - Cancel downloads at any time (terminates yt-dlp process)
 - Per-user quota system (daily, weekly, monthly; 0 = unlimited)