Phase 7: Undo/Redo

- History tracking with 50-state limit in useDesignEditor hook
- Undo: Ctrl/Cmd + Z keyboard shortcut
- Redo: Ctrl/Cmd + Shift + Z or Ctrl/Cmd + Y
- Undo/Redo buttons in canvas header
- History saves state after add, update, delete, reorder operations
- Disabled button states when history is exhausted

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

client/src/index.css

@@ -102,16 +102,57 @@ input, textarea, select {
 .app-title {
   font-size: 1.5rem;
   font-weight: 600;
-  margin: 0 0 0.5rem 0;
+  margin: 0 0 0.25rem 0;
   color: var(--text-primary);
 }
 
 .app-subtitle {
   color: var(--text-secondary);
-  margin: 0 0 1.5rem 0;
+  margin: 0;
   font-size: 0.875rem;
 }
 
+.canvas-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: flex-start;
+  margin-bottom: 1rem;
+  width: 100%;
+  max-width: 400px;
+}
+
+.undo-redo-buttons {
+  display: flex;
+  gap: 0.5rem;
+}
+
+.icon-btn {
+  padding: 0.5rem 0.75rem;
+  background: var(--bg-primary);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  font-size: 0.75rem;
+  cursor: pointer;
+  transition: all 0.2s;
+  color: var(--text-secondary);
+}
+
+.icon-btn:hover:not(:disabled) {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: white;
+}
+
+.icon-btn.disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+
+.icon-btn:disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+
 .canvas-wrapper {
   margin-bottom: 1rem;
 }