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>

components/layers/index.ts

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+export * from './layers-panel';

components/layers/layers-panel.tsx

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+'use client';
+
+import { useLayerStore } from '@/store';
+import { Eye, EyeOff, Trash2 } from 'lucide-react';
+import { cn } from '@/lib/utils';
+
+export function LayersPanel() {
+  const { layers, activeLayerId, setActiveLayer, updateLayer, deleteLayer } = useLayerStore();
+
+  // Sort layers by order (highest first)
+  const sortedLayers = [...layers].sort((a, b) => b.order - a.order);
+
+  return (
+    <div className="flex h-full flex-col bg-card">
+      <div className="border-b border-border p-3">
+        <h2 className="text-sm font-semibold text-card-foreground">Layers</h2>
+      </div>
+
+      <div className="flex-1 overflow-y-auto p-2 space-y-1">
+        {sortedLayers.length === 0 ? (
+          <div className="flex h-full items-center justify-center">
+            <p className="text-sm text-muted-foreground">No layers</p>
+          </div>
+        ) : (
+          sortedLayers.map((layer) => (
+            <div
+              key={layer.id}
+              className={cn(
+                'group flex items-center gap-2 rounded-md border p-2 transition-colors cursor-pointer',
+                activeLayerId === layer.id
+                  ? 'border-primary bg-primary/10'
+                  : 'border-border hover:border-primary/50 hover:bg-accent/50'
+              )}
+              onClick={() => setActiveLayer(layer.id)}
+            >
+              <button
+                className="shrink-0 text-muted-foreground hover:text-foreground"
+                onClick={(e) => {
+                  e.stopPropagation();
+                  updateLayer(layer.id, { visible: !layer.visible });
+                }}
+              >
+                {layer.visible ? (
+                  <Eye className="h-4 w-4" />
+                ) : (
+                  <EyeOff className="h-4 w-4" />
+                )}
+              </button>
+
+              <div className="flex-1 min-w-0">
+                <p className="text-sm font-medium text-card-foreground truncate">
+                  {layer.name}
+                </p>
+                <p className="text-xs text-muted-foreground">
+                  {layer.width} × {layer.height}
+                </p>
+              </div>
+
+              <div className="flex items-center gap-1 opacity-0 group-hover:opacity-100 transition-opacity">
+                <button
+                  className="shrink-0 text-muted-foreground hover:text-destructive"
+                  onClick={(e) => {
+                    e.stopPropagation();
+                    if (confirm('Delete this layer?')) {
+                      deleteLayer(layer.id);
+                    }
+                  }}
+                >
+                  <Trash2 className="h-4 w-4" />
+                </button>
+              </div>
+
+              <div className="shrink-0 text-xs text-muted-foreground">
+                {Math.round(layer.opacity * 100)}%
+              </div>
+            </div>
+          ))
+        )}
+      </div>
+    </div>
+  );
+}