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

ViewModels/MainViewModel.cs

@@ -177,6 +177,7 @@ public class MainViewModel : ViewModelBase
                 OnPropertyChanged(nameof(CanChangeBackdrop));
                 OnPropertyChanged(nameof(CanAddWebcamToActiveScene));
                 OnPropertyChanged(nameof(CanShowWebcamInActiveScene));
+                OnPropertyChanged(nameof(IsCountdownAvailable));
                 UpdateActiveBackground();
                 UpdateBackdropImage();
             }
@@ -729,6 +730,9 @@ public class MainViewModel : ViewModelBase
     public bool ShowPreviewPlaceholder => !ShowEmptySceneHint && ActiveBackgroundImage == null && BackdropImage == null;
     public bool ShowSourcesEmptyHint => ActiveScene == null || ActiveScene.Elements.Count == 0;
 
+    public bool IsCountdownAvailable => ActiveScene != null
+        && (SceneCatalog.Is(ActiveScene.Name, "Starting") || SceneCatalog.Is(ActiveScene.Name, "BRB"));
+
     /// <summary>The canonical scenes (SceneCatalog) not present right now — what the
     /// "+" button may re-add. Work with less, never more.</summary>
     public IEnumerable<string> MissingScenes
@@ -1721,6 +1725,8 @@ public class MainViewModel : ViewModelBase
             SourceType.Background => "Background",
             SourceType.Image => "Image",
             SourceType.TextOverlay => "Text",
+            SourceType.Countdown => "Countdown",
+            SourceType.WebSource => "Web",
             _ => "Source",
         };