Phase 3: Sidebar & Properties Panel

- Three-column layout (sidebar/canvas/properties)
- Sidebar with tabs: Upload, Stickers, Text
- Upload tab with drag-and-drop and click-to-upload
- Stickers tab with 6 categories (40+ emojis)
- Text tab with font selector, size slider, color picker
- Properties panel with position, size, rotation controls
- Delete button for selected element
- Responsive layout for mobile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/components/properties/index.js

@@ -0,0 +1 @@
+export { PropertiesPanel } from './PropertiesPanel';