Polish UI flows and unify API handling

[leoz9]

Summary

• Improve frontend UX/UI flows with low-risk updates across Entry, Wardrobe, Outfit, Recommendation, Home, and TabBar (mobile-first clarity, actionable empty states, smoother interactions).
• Centralize frontend API/image URL building through frontend/src/utils/api.js and migrate pages/contexts to remove duplicate hardcoded URL logic.
• Apply lightweight backend optimizations without API contract changes:
  • cache config reads by file mtime in backend/storage/config_store.py
  • streamline wardrobe/category query path in backend/api/wardrobe.py
  • reduce repeated category normalization in backend/services/recommendation.py
• Refresh README (EN/ZH/JA) with new UI screenshot layout, add GitHub stars badge, and fix Star History chart parameter consistency (type=Date).

Test plan

• Frontend build passes: cd frontend && npm run build
• Changed backend modules compile: python3 -m py_compile backend/storage/config_store.py backend/api/wardrobe.py backend/services/recommendation.py
• Manual smoke test on key pages: Entry / Wardrobe / Outfit / Recommendation / Home
• Verify README rendering on GitHub (images, badges, star chart)