feat: Elements panel, Countdown/Web sources, webcam border config (TASK 14 Branch 2)

- New SourceType.Countdown (Starting/BRB only, timer 1-60 min) and SourceType.WebSource (all scenes, URI)
- Elements panel beneath Sources: per-element config with live preview, ✕ revert / ✓ applied indicator
- Webcam border: hex color input + swatch picker, thickness 0-10 slider
- Countdown: timer minutes slider (1-60, default 15), Web: URI text input
- SourceTypeToVisibilityConverter for conditional panel visibility
- Snapshot/revert mechanism in code-behind for live-preview cancel

Helpers/SourceTypeToVisibilityConverter.cs

@@ -0,0 +1,21 @@
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+using ytLive.Models;
+
+namespace ytLive.Helpers;
+
+public class SourceTypeToVisibilityConverter : IValueConverter
+{
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+    {
+        if (value is not SourceType current || parameter is not string targetName)
+            return Visibility.Collapsed;
+        return Enum.TryParse<SourceType>(targetName, true, out var target) && current == target
+            ? Visibility.Visible
+            : Visibility.Collapsed;
+    }
+
+    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}