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