Mic audio UX: realtime read-only meter + mic picker

- Meter is a READ-ONLY realtime level display: fill = Math.Min(1, AudioLevel * MicVolume) (AudioLevel = live input from the future mixer, 0 with no input; MicVolume is gain on ambient noise). While the slider is dragged the bar previews the slider position (PreviewMouseLeftButtonDown/Up + LostMouseCapture -> SetVolumeAdjusting); on release it returns to the live level (bounces to 0 with no input). Clicking the meter does nothing.
- MicVolume drives MicMuted (read-only, muted <=> volume 0): sliding to 0 flips the speaker to the red slashed mute icon, sliding up from 0 clears it; speaker button runs volume to 0 or restores the prior level (_volumeBeforeMute, default 0.8) and flashes the meter to the restored position for ~300ms (BeginVolumeFlash/EndVolumeFlash DispatcherTimer, cancelled if the slider is grabbed).
- MIC label opens MicPickerDialog (WinRT audio-capture device enumeration, mirrors camera picker, no new packages); the chosen voice source name shows left-justified INSIDE the meter bar (fill at 75% opacity so text + ruler markings show through).
- New files: Services/IMicrophoneEnumerator, MicrophoneDeviceInfo, WinRtMicrophoneEnumerator; ViewModels/MicPickerViewModel; MicPickerDialog.
- Docs updated (ai.md, TASKS.md, Services/index.md, ViewModels/index.md); build 0 warnings, 65 tests passing

MainWindow.xaml.cs

@@ -81,6 +81,38 @@ public partial class MainWindow : Window
         }
     }
 
+    private void MicLabel_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
+    {
+        if (sender is FrameworkElement { DataContext: MainViewModel vm })
+            vm.OpenMicPickerCommand.Execute(null);
+    }
+
+    private void VolumeSlider_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
+    {
+        if (sender is FrameworkElement { DataContext: MainViewModel vm })
+            vm.SetVolumeAdjusting(true);
+    }
+
+    private void VolumeSlider_PreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
+    {
+        if (sender is FrameworkElement { DataContext: MainViewModel vm })
+            vm.SetVolumeAdjusting(false);
+    }
+
+    private void VolumeSlider_LostMouseCapture(object sender, MouseEventArgs e)
+    {
+        if (sender is FrameworkElement { DataContext: MainViewModel vm })
+            vm.SetVolumeAdjusting(false);
+    }
+
+    private void MicSpeaker_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
+    {
+        if (sender is FrameworkElement { DataContext: MainViewModel vm })
+        {
+            vm.ToggleMicMuteCommand.Execute(null);
+        }
+    }
+
     private void GearButton_Click(object sender, RoutedEventArgs e)
     {
         if (sender is Button { ContextMenu: { } menu } button)