Phase 3: Sidebar & Properties Panel

Implemented full editor UI with three-column layout:
- Sidebar with 4 tabs (Upload, Stickers, Text, Templates)
- UploadTab with drag-and-drop file upload, wires to POST /api/upload
- StickersTab with 96 emoji stickers across 6 categories
- TextTab with font picker (20 Google Fonts), size slider, color picker
- TemplatesTab placeholder for future template system
- LayersPanel showing all elements with select/delete
- PropertiesPanel with position, size, rotation controls

Also added:
- Constants for fonts and stickers
- Enhanced CSS with editor-layout, sidebar, properties-panel classes
- Updated App.jsx to integrate all components

client/src/components/sidebar/index.js

@@ -0,0 +1,5 @@
+export { Sidebar } from './Sidebar';
+export { UploadTab } from './UploadTab';
+export { StickersTab } from './StickersTab';
+export { TextTab } from './TextTab';
+export { TemplatesTab } from './TemplatesTab';