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

internal/domain/types.go

@@ -43,17 +43,14 @@ type Format struct {
 
 // MediaInfo holds the full information about a downloadable media item.
 type MediaInfo struct {
-	URL             string
-	Title           string
-	Uploader        string
-	Thumbnail       string
-	Duration        float64
-	Formats         []Format
-	Languages       []string
-	IsPlaylist      bool
-	PlaylistCount   int
-	PlaylistEntries []PlaylistEntry
-	EstimatedSize   int64
+	URL           string
+	Title         string
+	Uploader      string
+	Thumbnail     string
+	Duration      float64
+	Formats       []Format
+	Languages     []string
+	EstimatedSize int64
 }
 
 // IsAudioOnlyContent returns true when no format carries a video stream.
@@ -172,14 +169,6 @@ type DownloadRecord struct {
 	CreatedAt string
 }
 
-// PlaylistState tracks the user's current playlist selection flow.
-type PlaylistState struct {
-	Entries  []PlaylistEntry
-	Page     int
-	PerPage  int
-	Selected map[string]bool
-}
-
 // SearchState tracks the user's current search results flow.
 type SearchState struct {
 	Query   string