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/index.css

@@ -56,11 +56,54 @@ body {
 
 #root {
   min-height: 100vh;
+  display: flex;
+  flex-direction: column;
+}
+
+/* Three-column layout */
+.editor-layout {
+  display: flex;
+  flex: 1;
+  overflow: hidden;
+}
+
+.sidebar {
+  width: 320px;
+  background: var(--bg-secondary);
+  border-right: 1px solid var(--border);
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+}
+
+.canvas-area {
+  flex: 1;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  background: var(--bg-tertiary);
+  overflow: auto;
+  padding: 2rem;
+}
+
+.properties-panel {
+  width: 280px;
+  background: var(--bg-secondary);
+  border-left: 1px solid var(--border);
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
 }
 
 button {
   font-family: inherit;
   cursor: pointer;
+  outline: none;
+}
+
+button:focus-visible {
+  box-shadow: 0 0 0 2px var(--bg-primary), 0 0 0 4px var(--accent);
 }
 
 input, textarea, select {