Add real-time CAVA audio visualizer for VU bars

CAVA runs in background via cava-start.sh, writing bar values to
/tmp/skeledance_bars. A new QML DataSource polls that file at 80ms
and calls updateBars() in JS. Falls back to rest state when not playing.

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

plugin/contents/ui/main.qml

@@ -24,6 +24,14 @@ WallpaperItem {
         }
     }
 
+    // Start CAVA on load
+    Plasma5Support.DataSource {
+        id: cavaStarter
+        engine: "executable"
+        connectedSources: ["bash " + Qt.resolvedUrl("cava-start.sh").toString().replace("file://", "") + " &"]
+        interval: 0
+    }
+
     // Polls playerctl every 2s and injects music info into the page
     Plasma5Support.DataSource {
         id: musicSource
@@ -42,4 +50,19 @@ WallpaperItem {
             )
         }
     }
+
+    // Polls CAVA bars at ~60fps
+    Plasma5Support.DataSource {
+        id: cavaSource
+        engine: "executable"
+        connectedSources: ["cat /tmp/skeledance_bars 2>/dev/null || echo ''"]
+        interval: 80
+
+        onNewData: function(sourceName, data) {
+            var raw = data["stdout"].trim()
+            if (raw !== "") {
+                webView.runJavaScript("updateBars(" + JSON.stringify(raw) + ")")
+            }
+        }
+    }
 }