feat(ui): rename panels Screens → Layers → Properties

Renames all user-facing UI text to distinguish from OBS terminology:
- SCENES → SCREENS (section header, tooltips, hints)
- SOURCES → LAYERS (section header, tooltips, context menu, empty hints, onboarding text)
- ELEMENTS → PROPERTIES (section header, tooltips, empty hint)
- Fallback display name 'Source' → 'Layer'

Code identifiers unchanged (IsElementsPanelEmpty, ShowSourcesEmptyHint, etc.)
16 UI text locations across MainWindow.xaml + MainViewModel.cs

ViewModels/MainViewModel.cs

@@ -1796,7 +1796,7 @@ public class MainViewModel : ViewModelBase
             SourceType.Countdown => "Countdown",
             SourceType.WebSource => "Web",
             SourceType.ChatBox => "Chat",
-            _ => "Source",
+            _ => "Layer",
         };
 
         if (sourceType == SourceType.Background)