Add keyboard-activated Focus Mode to provide an immersive writing experience:

Features:
- Toggle with Cmd/Ctrl+Shift+F (matches GitHub, Google Docs)
- Exit with Escape, toggle shortcut, button click, or backdrop click
- Expands editor to ~80-90% of viewport with centered layout
- Semi-transparent backdrop with blur effect
- Maintains all editor functionality (attachments, shortcuts, etc.)
- Smooth 300ms transitions

Responsive Design:
- Mobile (< 640px): 8px margins, 50vh min-height
- Tablet (640-768px): 16px margins
- Desktop (> 768px): 32px margins, 60vh min-height, 1024px max-width

Implementation:
- Centralized constants for easy maintenance (FOCUS_MODE_STYLES)
- Extracted keyboard shortcuts and heights to named constants
- JSDoc documentation for all new functions and interfaces
- TypeScript type safety with 'as const'
- Explicit positioning (top/left/right/bottom) to avoid width overflow

Files Modified:
- web/src/components/MemoEditor/index.tsx - Main Focus Mode logic
- web/src/components/MemoEditor/Editor/index.tsx - Height adjustments
- web/src/locales/en.json - Translation keys

Design follows industry standards (GitHub Focus Mode, Notion, Obsidian)
and maintains code quality with single source of truth pattern.