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>

store/layer-store.ts

@@ -30,6 +30,16 @@ interface LayerStore {
   getActiveLayer: () => Layer | undefined;
   /** Clear all layers */
   clearLayers: () => void;
+  /** Add mask to layer */
+  addMask: (id: string) => void;
+  /** Remove mask from layer */
+  removeMask: (id: string) => void;
+  /** Enable/disable mask */
+  toggleMask: (id: string) => void;
+  /** Invert mask */
+  invertMask: (id: string) => void;
+  /** Apply mask (merge and remove) */
+  applyMask: (id: string) => void;
 }
 
 export const useLayerStore = create<LayerStore>((set, get) => ({
@@ -51,6 +61,7 @@ export const useLayerStore = create<LayerStore>((set, get) => ({
       height: params.height,
       x: params.x ?? 0,
       y: params.y ?? 0,
+      mask: null,
       createdAt: now,
       updatedAt: now,
     };
@@ -134,6 +145,22 @@ export const useLayerStore = create<LayerStore>((set, get) => ({
       newLayer.canvas = canvas;
     }
 
+    // Clone mask if exists
+    if (layer.mask?.canvas) {
+      const maskCanvas = document.createElement('canvas');
+      maskCanvas.width = layer.width;
+      maskCanvas.height = layer.height;
+      const maskCtx = maskCanvas.getContext('2d');
+      if (maskCtx) {
+        maskCtx.drawImage(layer.mask.canvas, 0, 0);
+      }
+      newLayer.mask = {
+        canvas: maskCanvas,
+        enabled: layer.mask.enabled,
+        inverted: layer.mask.inverted,
+      };
+    }
+
     set((state) => ({
       layers: [...state.layers, newLayer],
       activeLayerId: newLayer.id,
@@ -236,4 +263,91 @@ export const useLayerStore = create<LayerStore>((set, get) => ({
   clearLayers: () => {
     set({ layers: [], activeLayerId: null });
   },
+
+  addMask: (id) => {
+    const layer = get().getLayer(id);
+    if (!layer || layer.mask) return;
+
+    // Create mask canvas (white = fully visible)
+    const maskCanvas = document.createElement('canvas');
+    maskCanvas.width = layer.width;
+    maskCanvas.height = layer.height;
+    const ctx = maskCanvas.getContext('2d');
+    if (ctx) {
+      ctx.fillStyle = '#ffffff';
+      ctx.fillRect(0, 0, maskCanvas.width, maskCanvas.height);
+    }
+
+    get().updateLayer(id, {
+      mask: {
+        canvas: maskCanvas,
+        enabled: true,
+        inverted: false,
+      },
+    });
+  },
+
+  removeMask: (id) => {
+    get().updateLayer(id, { mask: null });
+  },
+
+  toggleMask: (id) => {
+    const layer = get().getLayer(id);
+    if (!layer?.mask) return;
+
+    get().updateLayer(id, {
+      mask: {
+        ...layer.mask,
+        enabled: !layer.mask.enabled,
+      },
+    });
+  },
+
+  invertMask: (id) => {
+    const layer = get().getLayer(id);
+    if (!layer?.mask?.canvas) return;
+
+    const maskCtx = layer.mask.canvas.getContext('2d');
+    if (!maskCtx) return;
+
+    // Invert the mask canvas
+    const imageData = maskCtx.getImageData(0, 0, layer.mask.canvas.width, layer.mask.canvas.height);
+    for (let i = 0; i < imageData.data.length; i += 4) {
+      imageData.data[i] = 255 - imageData.data[i];     // R
+      imageData.data[i + 1] = 255 - imageData.data[i + 1]; // G
+      imageData.data[i + 2] = 255 - imageData.data[i + 2]; // B
+    }
+    maskCtx.putImageData(imageData, 0, 0);
+
+    get().updateLayer(id, {
+      mask: {
+        ...layer.mask,
+        inverted: !layer.mask.inverted,
+      },
+    });
+  },
+
+  applyMask: (id) => {
+    const layer = get().getLayer(id);
+    if (!layer?.canvas || !layer.mask?.canvas) return;
+
+    const ctx = layer.canvas.getContext('2d');
+    const maskCtx = layer.mask.canvas.getContext('2d');
+    if (!ctx || !maskCtx) return;
+
+    // Get layer and mask data
+    const layerData = ctx.getImageData(0, 0, layer.width, layer.height);
+    const maskData = maskCtx.getImageData(0, 0, layer.width, layer.height);
+
+    // Apply mask to layer alpha channel
+    for (let i = 0; i < layerData.data.length; i += 4) {
+      const maskValue = maskData.data[i]; // Use red channel of mask
+      layerData.data[i + 3] = (layerData.data[i + 3] * maskValue) / 255;
+    }
+
+    ctx.putImageData(layerData, 0, 0);
+
+    // Remove mask
+    get().removeMask(id);
+  },
 }));