fix(MemoEditor): restore focus mode functionality

- Add useFocusMode hook to lock body scroll in focus mode
- Add 'f' key keyboard shortcut to toggle focus mode
- Add FocusModeExitButton inside editor
- Import and wire up all focus mode components and hooks
- Update useKeyboard to handle focus mode toggle

web/src/components/MemoEditor/hooks/useKeyboard.ts

 import { useEffect } from "react";
+import { FOCUS_MODE_TOGGLE_KEY } from "../constants";
 import type { EditorRefActions } from "../Editor";
 
 interface UseKeyboardOptions {
   onSave: () => void;
+  onToggleFocusMode?: () => void;
 }
 
 export const useKeyboard = (editorRef: React.RefObject<EditorRefActions | null>, options: UseKeyboardOptions) => {
@@ -12,6 +14,17 @@ export const useKeyboard = (editorRef: React.RefObject<EditorRefActions | null>,
       if ((event.metaKey || event.ctrlKey) && event.key === "Enter") {
         event.preventDefault();
         options.onSave();
+        return;
+      }
+
+      // 'f' key to toggle focus mode (only if no modifiers and not in input)
+      if (event.key === FOCUS_MODE_TOGGLE_KEY && !event.metaKey && !event.ctrlKey && !event.altKey && !event.shiftKey) {
+        const target = event.target as HTMLElement;
+        // Don't trigger if user is typing in an input/textarea
+        if (target.tagName !== "INPUT" && target.tagName !== "TEXTAREA" && !target.isContentEditable) {
+          event.preventDefault();
+          options.onToggleFocusMode?.();
+        }
       }
     };
 

web/src/components/MemoEditor/index.tsx

@@ -4,10 +4,10 @@ import { toast } from "react-hot-toast";
 import useCurrentUser from "@/hooks/useCurrentUser";
 import { cn } from "@/lib/utils";
 import { useTranslate } from "@/utils/i18n";
-import { EditorContent, EditorMetadata, EditorToolbar, FocusModeOverlay } from "./components";
+import { EditorContent, EditorMetadata, EditorToolbar, FocusModeExitButton, FocusModeOverlay } from "./components";
 import { FOCUS_MODE_STYLES } from "./constants";
 import type { EditorRefActions } from "./Editor";
-import { useAutoSave, useKeyboard, useMemoInit } from "./hooks";
+import { useAutoSave, useFocusMode, useKeyboard, useMemoInit } from "./hooks";
 import { cacheService, errorService, memoService, validationService } from "./services";
 import { EditorProvider, useEditorContext } from "./state";
 import { MemoEditorContext } from "./types";
@@ -80,8 +80,11 @@ const MemoEditorImpl: React.FC<Props> = ({
   // Auto-save content to localStorage
   useAutoSave(state.content, currentUser.name, cacheKey);
 
+  // Focus mode management with body scroll lock
+  useFocusMode(state.ui.isFocusMode);
+
   // Keyboard shortcuts
-  useKeyboard(editorRef, { onSave: handleSave });
+  useKeyboard(editorRef, { onSave: handleSave, onToggleFocusMode: actions.toggleFocusMode });
 
   async function handleSave() {
     const { valid, reason } = validationService.canSave(state);
@@ -131,6 +134,7 @@ const MemoEditorImpl: React.FC<Props> = ({
           className,
         )}
       >
+        <FocusModeExitButton isActive={state.ui.isFocusMode} onToggle={actions.toggleFocusMode} title={t("editor.exit-focus-mode")} />
         <EditorContent ref={editorRef} placeholder={placeholder} autoFocus={autoFocus} />
         <EditorMetadata />
         <EditorToolbar onSave={handleSave} onCancel={onCancel} />