feat: enhance layer masking functionality and brush controls

- Refactor LayersSheet component to support mask editing state and improve layer visibility handling.
- Introduce functions to collect mask layer IDs and count display layers excluding masks.
- Update BrushControls to include mask view mode options and a Done button for exiting mask editing.
- Modify brush session handling to support brush previews when editing masks.
- Implement a new BrushPreviewRenderer for rendering brush strokes with visual feedback.
- Add document geometry utilities for transforming points and resolving layer bounds.
- Ensure proper cleanup of brush preview on pointer leave and other interactions.

editor/initial-state.ts

@@ -13,7 +13,9 @@ export const initialEditorState: EditorState = {
   },
   tools: initialToolState,
   transformSession: undefined,
-  maskEditLayerId: undefined,
+  maskEdit: undefined,
+  brushPreview: undefined,
+  brushStrokePreview: undefined,
 };
 
 export function createInitialAppState(name = "Untitled"): AppState {