feat(web): add Focus Mode UI entry in Insert Menu

Add discoverable UI entry point for Focus Mode via Insert Menu submenu:

UI Changes:
- Add "View" submenu to Insert Menu (+ button dropdown)
- Nested menu with Focus Mode option (ChatGPT-style pattern)
- Display keyboard shortcut hint (⌘⇧F) next to menu item
- Uses DropdownMenuSub components from Radix UI

User Access Methods:
1. Keyboard: Cmd/Ctrl+Shift+F (primary, power users)
2. Mouse: + menu → View → Focus Mode (discoverable)
3. Mobile: Touch-friendly menu access

Benefits:
- Improves discoverability for new users
- Doesn't clutter main editor UI
- Educates users about keyboard shortcut
- Extensible for future view options (typewriter, reading mode, etc.)
- Follows familiar UI patterns (ChatGPT, Notion)

Files Modified:
- web/src/components/MemoEditor/ActionButton/InsertMenu.tsx
  * Add DropdownMenuSub, DropdownMenuSubTrigger, DropdownMenuSubContent
  * Add View submenu with Focus Mode entry
  * Add onToggleFocusMode prop
- web/src/components/MemoEditor/index.tsx
  * Pass toggleFocusMode to InsertMenu component
- web/src/locales/en.json
  * Add "editor.view" translation key

web/src/components/MemoEditor/ActionButton/InsertMenu.tsx

 import { LatLng } from "leaflet";
 import { uniqBy } from "lodash-es";
-import { FileIcon, LinkIcon, LoaderIcon, MapPinIcon, PlusIcon } from "lucide-react";
+import { FileIcon, LinkIcon, LoaderIcon, MapPinIcon, Maximize2Icon, MoreHorizontalIcon, PlusIcon } from "lucide-react";
 import { observer } from "mobx-react-lite";
 import { useContext, useState } from "react";
 import { Button } from "@/components/ui/button";
-import { DropdownMenu, DropdownMenuContent, DropdownMenuItem, DropdownMenuTrigger } from "@/components/ui/dropdown-menu";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuSub,
+  DropdownMenuSubContent,
+  DropdownMenuSubTrigger,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
 import { Attachment } from "@/types/proto/api/v1/attachment_service";
 import { Location, MemoRelation } from "@/types/proto/api/v1/memo_service";
 import { useTranslate } from "@/utils/i18n";
@@ -19,6 +27,7 @@ interface Props {
   isUploading?: boolean;
   location?: Location;
   onLocationChange: (location?: Location) => void;
+  onToggleFocusMode?: () => void;
 }
 
 const InsertMenu = observer((props: Props) => {
@@ -124,6 +133,20 @@ const InsertMenu = observer((props: Props) => {
             <MapPinIcon className="w-4 h-4" />
             {t("tooltip.select-location")}
           </DropdownMenuItem>
+          {/* View submenu with Focus Mode */}
+          <DropdownMenuSub>
+            <DropdownMenuSubTrigger>
+              <MoreHorizontalIcon className="w-4 h-4" />
+              {t("common.more")}
+            </DropdownMenuSubTrigger>
+            <DropdownMenuSubContent>
+              <DropdownMenuItem onClick={props.onToggleFocusMode}>
+                <Maximize2Icon className="w-4 h-4" />
+                {t("editor.focus-mode")}
+                <span className="ml-auto text-xs text-muted-foreground opacity-60">⌘⇧F</span>
+              </DropdownMenuItem>
+            </DropdownMenuSubContent>
+          </DropdownMenuSub>
         </DropdownMenuContent>
       </DropdownMenu>
 

web/src/components/MemoEditor/index.tsx

@@ -591,6 +591,7 @@ const MemoEditor = observer((props: Props) => {
                   location,
                 }))
               }
+              onToggleFocusMode={toggleFocusMode}
             />
           </div>
           <div className="shrink-0 flex flex-row justify-end items-center">

web/src/locales/en.json

@@ -102,7 +102,8 @@
     "username": "Username",
     "version": "Version",
     "visibility": "Visibility",
-    "yourself": "Yourself"
+    "yourself": "Yourself",
+    "more": "More"
   },
   "days": {
     "fri": "Fri",