feat(ui/perf): implement loading states, keyboard navigation, and lazy-loaded tools

Add comprehensive UX and performance improvements:

**Loading States & Feedback:**
- Add loading overlay with spinner and custom messages
- Integrate loading states into all file operations (open, save, export)
- Create loading-store.ts for centralized loading state management

**Keyboard Navigation:**
- Expand keyboard shortcuts to include tool selection (1-7)
- Add layer navigation with Arrow Up/Down
- Add layer operations (Ctrl+D duplicate, Delete/Backspace remove)
- Display keyboard shortcuts in tool tooltips
- Enhanced keyboard shortcut system with proper key conflict handling

**Performance - Code Splitting:**
- Implement dynamic tool loader with lazy loading
- Tools load on-demand when first selected
- Preload common tools (pencil, brush, eraser) for instant access
- Add tool caching to prevent redundant loads
- Reduces initial bundle size and improves startup time

**Integration:**
- Add LoadingOverlay to app layout
- Update canvas-with-tools to use lazy-loaded tool instances
- Add keyboard shortcut hints to tool palette UI

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

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

components/ui/loading-overlay.tsx

@@ -0,0 +1,29 @@
+'use client';
+
+import { useLoadingStore } from '@/store/loading-store';
+import { Loader2 } from 'lucide-react';
+
+export function LoadingOverlay() {
+  const { isLoading, loadingMessage } = useLoadingStore();
+
+  if (!isLoading) return null;
+
+  return (
+    <div
+      className="fixed inset-0 z-[10000] bg-background/80 backdrop-blur-sm flex items-center justify-center animate-fadeIn"
+      role="dialog"
+      aria-busy="true"
+      aria-label="Loading"
+    >
+      <div className="bg-card border border-border rounded-lg p-6 shadow-2xl flex flex-col items-center gap-4 min-w-[300px]">
+        <Loader2 className="h-12 w-12 text-primary animate-spin" aria-hidden="true" />
+        {loadingMessage && (
+          <p className="text-sm font-medium text-foreground text-center">
+            {loadingMessage}
+          </p>
+        )}
+        <p className="text-xs text-muted-foreground">Please wait...</p>
+      </div>
+    </div>
+  );
+}