feat(undo): Add keyboard shortcuts (Cmd/Ctrl+Z)

Author: AmirMohammad Cheraghali

src/App.tsx

@@ -363,6 +363,29 @@ function App() {
     resetTrigger: pdbId
   });
 
+  // Keyboard Shortcuts for Undo/Redo
+  useEffect(() => {
+    const handleKeyDown = (e: KeyboardEvent) => {
+      // Ignore if typing in an input or textarea
+      if (e.target instanceof HTMLInputElement || e.target instanceof HTMLTextAreaElement) {
+        return;
+      }
+
+      const isMod = e.metaKey || e.ctrlKey;
+      if (isMod && e.key.toLowerCase() === 'z') {
+        e.preventDefault();
+        if (e.shiftKey) {
+          if (canRedo) redo();
+        } else {
+          if (canUndo) undo();
+        }
+      }
+    };
+
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, [undo, redo, canUndo, canRedo]);
+
   // --- DERIVED STATE (Dr. AI V4) ---
   const structureStats = useMemo(() => {
     const chainCount = chains.length;