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/get-playing.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+for player in $(playerctl -l 2>/dev/null); do
+    if [ "$(playerctl --player "$player" status 2>/dev/null)" = "Playing" ]; then
+        playerctl --player "$player" metadata --format "{{status}}||{{artist}}||{{title}}" 2>/dev/null
+        exit 0
+    fi
+done
+printf "||"