docs: update README to production-ready English-only format

- Remove all development phase sections and implementation progress
- Translate all Japanese content to English
- Update version numbers to match package.json:
  * Next.js 16.0.1 → 16.0.3
  * next-intl 4.4.0 → 4.5.3
  * Framer Motion 11.15.0 → 12.23.24
  * Lucide React 0.468.0 → 0.553.0
  * Zustand 5.0.2 → 5.0.8
  * Add Node.js 22.21.1 version
- Restructure into user-focused format with status badges
- Remove references to IMPLEMENTATION_PHASE documents
- Consolidate features by functional area (Timer, Audio, Notifications, i18n, PWA, Accessibility)
- Add comprehensive tech stack with documentation links
- Reduce file size from 347 to 276 lines (20% reduction)
- Maintain design system and technical decision sections

Result: Clean, professional README appropriate for production use

Author: ryota-murakami