Implement offline mode

[Greyisheep]

📱 Implement offline mode

Description

Add service worker for offline functionality and cache essential data locally.

Tasks

• Add service worker
• Cache essential data
• Handle offline state
• Sync when online

Files to create

• frontend/public/sw.js
• frontend/src/services/offline.js

Acceptance Criteria

• Works offline
• Data is cached
• Sync works properly
• User experience is maintained

Labels

• hard
• frontend

Estimated Time

15-25 hours