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

MainWindow.xaml.cs

@@ -227,6 +227,7 @@ public partial class MainWindow : Window
         OpacityChip.Visibility = selected ? Visibility.Visible : Visibility.Collapsed;
         if (selected)
             OpacityValueText.Text = $"{Math.Round(_viewModel.SelectedElement!.Opacity * 100)}%";
+        TakeSnapshot();
     }
 
     // Static images and the webcam share the move/resize/selection behavior.
@@ -516,4 +517,73 @@ public partial class MainWindow : Window
         }
         return null;
     }
+
+    private Models.SceneElement? _snapshotElement;
+    private string _snapshotBorderColor = string.Empty;
+    private int _snapshotBorderWidth;
+    private int _snapshotTimerMinutes = 15;
+    private string? _snapshotWebUri;
+
+    private void TakeSnapshot()
+    {
+        if (_viewModel.SelectedElement is Models.Source src)
+        {
+            _snapshotElement = src;
+            _snapshotBorderColor = src.BorderColor;
+            _snapshotBorderWidth = src.BorderWidth;
+            _snapshotTimerMinutes = src.TimerMinutes;
+            _snapshotWebUri = src.WebUri;
+        }
+    }
+
+    private void WebcamBorderColorPicker_Click(object sender, RoutedEventArgs e)
+    {
+        if (_viewModel.SelectedElement is not Models.Source src) return;
+        if (sender is FrameworkElement fe && fe.ContextMenu == null)
+        {
+            var menu = new System.Windows.Controls.ContextMenu();
+            string[] colors = ["#FFFFFF", "#000000", "#FF0000", "#00FF00", "#0000FF",
+                               "#FFFF00", "#FF00FF", "#00FFFF", "#FF8800", "#8800FF",
+                               "#e94560", "#22c55e", "#3b82f6", "#f59e0b"];
+            foreach (var c in colors)
+            {
+                var item = new System.Windows.Controls.MenuItem { Header = c, ToolTip = c };
+                var brush = new System.Windows.Media.SolidColorBrush(
+                    (System.Windows.Media.Color)System.Windows.Media.ColorConverter.ConvertFromString(c));
+                item.Icon = new System.Windows.Controls.Border
+                {
+                    Width = 16, Height = 16,
+                    Background = brush,
+                    CornerRadius = new System.Windows.CornerRadius(2)
+                };
+                var color = c;
+                item.Click += (_, _) => src.BorderColor = color;
+                menu.Items.Add(item);
+            }
+            fe.ContextMenu = menu;
+        }
+        if (sender is FrameworkElement fe2)
+            fe2.ContextMenu.IsOpen = true;
+    }
+
+    private void WebcamBorderRevert_Click(object sender, RoutedEventArgs e)
+    {
+        if (_snapshotElement is Models.Source src)
+        {
+            src.BorderColor = _snapshotBorderColor;
+            src.BorderWidth = _snapshotBorderWidth;
+        }
+    }
+
+    private void CountdownRevert_Click(object sender, RoutedEventArgs e)
+    {
+        if (_snapshotElement is Models.Source src)
+            src.TimerMinutes = _snapshotTimerMinutes;
+    }
+
+    private void WebRevert_Click(object sender, RoutedEventArgs e)
+    {
+        if (_snapshotElement is Models.Source src)
+            src.WebUri = _snapshotWebUri;
+    }
 }