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>

store/loading-store.ts

@@ -0,0 +1,19 @@
+import { create } from 'zustand';
+
+interface LoadingState {
+  isLoading: boolean;
+  loadingMessage: string | null;
+  setLoading: (loading: boolean, message?: string) => void;
+}
+
+export const useLoadingStore = create<LoadingState>((set) => ({
+  isLoading: false,
+  loadingMessage: null,
+
+  setLoading: (loading: boolean, message?: string) => {
+    set({
+      isLoading: loading,
+      loadingMessage: message || null,
+    });
+  },
+}));