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