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/AudioLevelMeter.cs

@@ -1,18 +1,27 @@
 namespace ytLive.Services.Audio;
 
 /// <summary>
-/// Computes the smoothed mic level (0..1) from captured samples. Pure —
+/// Computes the smoothed level (0..1) from captured samples. Pure —
 /// unit-tested; the <see cref="AudioMixer"/> only feeds it and forwards the
 /// result. RMS-based so it tracks perceived loudness, smoothed so the meter
-/// does not flicker.
+/// does not flicker. Supports asymmetric smoothing: fast attack (rise) and
+/// configurable decay (fall) so the meter responds quickly to silence.
 /// </summary>
 public sealed class AudioLevelMeter
 {
-    private const float Smoothing = 0.2f;
     private float _level;
 
     public float Level => _level;
 
+    public AudioLevelMeter(float attackSmoothing = 0.2f, float decaySmoothing = 0.08f)
+    {
+        AttackSmoothing = attackSmoothing;
+        DecaySmoothing = decaySmoothing;
+    }
+
+    public float AttackSmoothing { get; }
+    public float DecaySmoothing { get; }
+
     /// <summary>
     /// Maps a linear level (0..1) onto the meter's display scale: -60 dBFS..0 dBFS
     /// spread linearly across 0..1, with +10 dB of input amplification. Linear RMS
@@ -44,7 +53,8 @@ public sealed class AudioLevelMeter
         }
 
         var rms = (float)Math.Sqrt(sumSquares / count);
-        _level = _level * (1 - Smoothing) + rms * Smoothing;
+        var smoothing = rms > _level ? AttackSmoothing : DecaySmoothing;
+        _level = _level * (1 - smoothing) + rms * smoothing;
         return _level;
     }