feat: implement Phase 2 - Core Canvas Engine with layer system

Complete canvas rendering infrastructure and state management:

**Type System (types/)**
- Layer interface with blend modes, opacity, visibility
- Canvas state with zoom, pan, grid, rulers
- Tool types and settings interfaces
- Selection and pointer state types

**State Management (store/)**
- Layer store: CRUD operations, reordering, merging, flattening
- Canvas store: zoom (0.1x-10x), pan, grid, rulers, coordinate conversion
- Tool store: active tool, brush settings (size, opacity, hardness, flow)
- Full Zustand integration with selectors

**Utilities (lib/)**
- Canvas utils: create, clone, resize, load images, draw grid/checkerboard
- General utils: cn, clamp, lerp, distance, snap to grid, debounce, throttle
- Image data handling with error safety

**Components**
- CanvasWrapper: Multi-layer rendering with transformations
  - Checkerboard transparency background
  - Layer compositing with blend modes and opacity
  - Grid overlay support
  - Selection visualization
  - Mouse wheel zoom (Ctrl+scroll)
  - Middle-click or Shift+click panning

- LayersPanel: Interactive layer management
  - Visibility toggle with eye icon
  - Active layer selection
  - Opacity display
  - Delete with confirmation
  - Sorted by z-order

- EditorLayout: Full editor interface
  - Top toolbar with zoom controls (±, fit to screen, percentage)
  - Canvas area with full viewport
  - Right sidebar for layers panel
  - "New Layer" button with auto-naming

**Features Implemented**
✓ Multi-layer canvas with proper z-ordering
✓ Layer visibility, opacity, blend modes
✓ Zoom: 10%-1000% with Ctrl+wheel
✓ Pan: Middle-click or Shift+drag
✓ Grid overlay (toggleable)
✓ Selection rendering
✓ Background color support
✓ Create/delete/duplicate layers
✓ Layer merging and flattening

**Performance**
- Dev server: 451ms startup
- Efficient canvas rendering with transformations
- Debounced/throttled event handlers ready
- Memory-safe image data operations

Ready for Phase 3: History & Undo System

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

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

types/layer.ts

@@ -0,0 +1,71 @@
+/**
+ * Blend modes for layer compositing
+ */
+export type BlendMode =
+  | 'normal'
+  | 'multiply'
+  | 'screen'
+  | 'overlay'
+  | 'darken'
+  | 'lighten'
+  | 'color-dodge'
+  | 'color-burn'
+  | 'hard-light'
+  | 'soft-light'
+  | 'difference'
+  | 'exclusion'
+  | 'hue'
+  | 'saturation'
+  | 'color'
+  | 'luminosity';
+
+/**
+ * Layer interface representing a single layer in the canvas
+ */
+export interface Layer {
+  /** Unique identifier for the layer */
+  id: string;
+  /** Display name of the layer */
+  name: string;
+  /** Canvas element containing the layer's image data */
+  canvas: HTMLCanvasElement | null;
+  /** Visibility state */
+  visible: boolean;
+  /** Opacity (0-1) */
+  opacity: number;
+  /** Blend mode for compositing */
+  blendMode: BlendMode;
+  /** Z-index order (higher = on top) */
+  order: number;
+  /** Lock state (prevents editing) */
+  locked: boolean;
+  /** Layer dimensions */
+  width: number;
+  height: number;
+  /** Position offset */
+  x: number;
+  y: number;
+  /** Timestamp of creation */
+  createdAt: number;
+  /** Timestamp of last modification */
+  updatedAt: number;
+}
+
+/**
+ * Partial layer data for updates
+ */
+export type LayerUpdate = Partial<Omit<Layer, 'id' | 'createdAt'>>;
+
+/**
+ * Layer creation parameters
+ */
+export interface CreateLayerParams {
+  name?: string;
+  width: number;
+  height: number;
+  x?: number;
+  y?: number;
+  opacity?: number;
+  blendMode?: BlendMode;
+  fillColor?: string;
+}