Fix UI polish: scrollbar deselect, web source inline buttons, countdown in Sources, volume click-to-position, meter decay

- Elements scrollbar no longer deselects element on click (check ScrollViewer, not StackPanel)
- Web source: URI + check/revert buttons merged into single DockPanel row, consistent order (✓ rightmost)
- Countdown timer moved from Elements panel to inline in source list row, range clamped to 1-30 min
- Volume slider click-to-position restored: e.Handled prevents IncreaseLarge from overriding position, Thumb drag still works
- Desktop audio meter: asymmetric smoothing (attack 0.2, decay 0.08) for faster response to silence
- 207 tests passing, 0 warnings

Models/Source.cs

@@ -96,7 +96,7 @@ public class Source : SceneElement
     public int TimerMinutes
     {
         get => _timerMinutes;
-        set => Set(ref _timerMinutes, Math.Clamp(value, 1, 60));
+        set => Set(ref _timerMinutes, Math.Clamp(value, 1, 30));
     }
 
     // Web source config