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

Models/Source.cs

@@ -9,7 +9,9 @@ public enum SourceType
     WindowCapture,
     Background,
     Image,
-    TextOverlay
+    TextOverlay,
+    Countdown,
+    WebSource
 }
 
 public enum ClipShape
@@ -86,4 +88,16 @@ public class Source : SceneElement
     public override ImageSource? DisplaySource => IsLiveCapture ? VideoImageSource : _imageSource;
 
     public override bool IsImageSource => Type == SourceType.Image;
+
+    // Countdown config
+    private int _timerMinutes = 15;
+    public int TimerMinutes
+    {
+        get => _timerMinutes;
+        set => Set(ref _timerMinutes, Math.Clamp(value, 1, 60));
+    }
+
+    // Web source config
+    private string? _webUri;
+    public string? WebUri { get => _webUri; set => Set(ref _webUri, value); }
 }