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>

app/layout.tsx

@@ -2,6 +2,7 @@ import type { Metadata } from 'next';
 import './globals.css';
 import { ToastProvider } from '@/components/providers/toast-provider';
 import { ContextMenu } from '@/components/ui/context-menu';
+import { LoadingOverlay } from '@/components/ui/loading-overlay';
 
 export const metadata: Metadata = {
   title: 'Paint UI - Browser Image Editor',
@@ -38,6 +39,7 @@ export default function RootLayout({
         {children}
         <ToastProvider />
         <ContextMenu />
+        <LoadingOverlay />
       </body>
     </html>
   );