Phases 7-10: Complete remaining features and optimizations

Phase 7.2 - Debounce undo/redo history:
- Add 300ms debounce timer for rapid drag/transform changes
- Commit history on dragEnd/transformEnd events only
- Prevents history bloat during continuous interactions

Phase 8.3 - Template-aware export:
- Render template background layer first
- Apply slot crop regions for image elements
- Render template overlay layer last
- Support nonPrintable flag for guides/watermarks

Phase 9 - PWA icons:
- Add pwa-192x192.svg and pwa-512x512.svg icons
- Update vite.config.js manifest configuration

Phase 10.3 - Performance optimizations:
- Add React.memo to canvas components (ImageElement, TextElement, DesignCanvas)
- Add React.memo to panel components (LayersPanel, PropertiesPanel)
- Prevent unnecessary re-renders during canvas interactions

Phase 10.6 - Template documentation:
- Document template JSON schema in docs/template-schema.md
- Include element properties, slot definitions, and examples
- Describe background/overlay layer structure

client/src/components/panels/PropertiesPanel.jsx

@@ -1,4 +1,6 @@
-export function PropertiesPanel({ element, onUpdate, onDelete }) {
+import { memo } from 'react';
+
+export const PropertiesPanel = memo(function PropertiesPanel({ element, onUpdate, onDelete }) {
   if (!element) {
     return (
       <div className="properties-panel">
@@ -278,4 +280,4 @@ export function PropertiesPanel({ element, onUpdate, onDelete }) {
       </div>
     </div>
   );
-}
+});