feat(phase-13): implement layer masks foundation for non-destructive editing

Add comprehensive layer mask system with core functionality for non-destructive editing.

Features:
- LayerMask interface with canvas, enabled, and inverted properties
- Layer interface updated with mask property
- Full mask management in layer store:
  * addMask() - Creates white mask (fully visible)
  * removeMask() - Removes mask from layer
  * toggleMask() - Enable/disable mask
  * invertMask() - Invert mask values
  * applyMask() - Apply mask to layer and remove
- Duplicate layer now clones masks
- Grayscale mask system (white=reveal, black=hide)

Changes:
- Updated types/layer.ts with LayerMask interface
- Modified Layer interface to include mask property
- Updated store/layer-store.ts:
  * Added mask property initialization to createLayer
  * Added 5 new mask management functions
  * Updated duplicateLayer to clone masks

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

types/layer.ts

@@ -19,6 +19,18 @@ export type BlendMode =
   | 'color'
   | 'luminosity';
 
+/**
+ * Layer mask data
+ */
+export interface LayerMask {
+  /** Canvas element containing the mask's grayscale data */
+  canvas: HTMLCanvasElement | null;
+  /** Whether the mask is currently enabled */
+  enabled: boolean;
+  /** Whether to invert the mask (white=hide, black=reveal) */
+  inverted: boolean;
+}
+
 /**
  * Layer interface representing a single layer in the canvas
  */
@@ -45,6 +57,8 @@ export interface Layer {
   /** Position offset */
   x: number;
   y: number;
+  /** Layer mask for non-destructive editing */
+  mask: LayerMask | null;
   /** Timestamp of creation */
   createdAt: number;
   /** Timestamp of last modification */