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

Services/Audio/AudioMixer.cs

@@ -57,7 +57,7 @@ public sealed class AudioMixer : IDisposable
         _mic = mic;
         _loopback = loopback;
         _meter = new AudioLevelMeter();
-        _loopbackMeter = new AudioLevelMeter();
+        _loopbackMeter = new AudioLevelMeter(attackSmoothing: 0.2f, decaySmoothing: 0.08f);
         _log = log;
         _micGain = micGain;
         _loopbackGain = loopbackGain;