feat: multi-player support + 5-line karaoke lyrics display

- get-playing.sh: iterates playerctl players, picks the one actually
  Playing; QML now calls this instead of bare playerctl metadata so
  a paused Spotify no longer blocks a playing YouTube from showing

- dancer.html: replace single lyric line with 5-slot karaoke display —
  active line (center) is full-brightness with theme-color glow; 2 lines
  before/after are progressively dimmed; slots fade out/in (0.18s/0.45s)
  when the active line advances

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

plugin/contents/ui/main.qml

@@ -38,7 +38,7 @@ WallpaperItem {
     Plasma5Support.DataSource {
         id: musicSource
         engine: "executable"
-        connectedSources: ["playerctl metadata --format '{{status}}||{{artist}}||{{title}}' 2>/dev/null || echo '||'"]
+        connectedSources: ["bash " + Qt.resolvedUrl("get-playing.sh").toString().replace("file://", "")]
         interval: 2000
 
         onNewData: function(sourceName, data) {