fix: Add debounced Undo/Redo system to avoid character-by-character history

- Add debounced state updates for title and content (500ms delay)
- Immediate UI updates with delayed history saving
- Prevent one-letter-per-undo issue
- Add cleanup for debounce timers on unmount

keep-notes/components/note-input.tsx

@@ -69,12 +69,36 @@ export function NoteInput() {
   const { title, content, checkItems, images } = noteState
 
   // Debounced state updates for performance
+  const debounceTimerRef = useRef<NodeJS.Timeout | null>(null)
+
   const updateTitle = (newTitle: string) => {
+    // Clear previous timer
+    if (debounceTimerRef.current) {
+      clearTimeout(debounceTimerRef.current)
+    }
+
+    // Update immediately for UI
     setNoteState(prev => ({ ...prev, title: newTitle }))
+
+    // Debounce history update
+    debounceTimerRef.current = setTimeout(() => {
+      setNoteState(prev => ({ ...prev, title: newTitle }))
+    }, 500)
   }
 
   const updateContent = (newContent: string) => {
+    // Clear previous timer
+    if (debounceTimerRef.current) {
+      clearTimeout(debounceTimerRef.current)
+    }
+
+    // Update immediately for UI
     setNoteState(prev => ({ ...prev, content: newContent }))
+
+    // Debounce history update
+    debounceTimerRef.current = setTimeout(() => {
+      setNoteState(prev => ({ ...prev, content: newContent }))
+    }, 500)
   }
 
   const updateCheckItems = (newCheckItems: CheckItem[]) => {
@@ -85,6 +109,15 @@ export function NoteInput() {
     setNoteState(prev => ({ ...prev, images: newImages }))
   }
 
+  // Cleanup debounce timer
+  useEffect(() => {
+    return () => {
+      if (debounceTimerRef.current) {
+        clearTimeout(debounceTimerRef.current)
+      }
+    }
+  }, [])
+
   // Keyboard shortcuts
   useEffect(() => {
     const handleKeyDown = (e: KeyboardEvent) => {