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/public/pwa-512x512.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="512" height="512" viewBox="0 0 512 512">
+  <rect width="512" height="512" fill="#38bdf8"/>
+  <rect x="64" y="64" width="384" height="384" rx="64" fill="#ffffff"/>
+  <text x="256" y="310" font-size="192" text-anchor="middle" fill="#38bdf8" font-family="Arial, sans-serif">T</text>
+</svg>