Implement offline support baseline (cache for feed/products/health)

[syed-reza98]

Why

Docs flag no offline support (except some care caching). This causes spinners/broken UX on poor connections.

Scope (baseline)

• Cache last successful responses locally (read-only offline)
• Define a cache invalidation strategy

Tasks

• Choose local store (Hive/Isar/SharedPreferences depending on complexity)
• Implement cache layer for:
  • Feed posts (last N)
  • Marketplace products
  • Health records (read-mostly)
• UI: show cached content with offline banner, allow manual refresh

Acceptance criteria

• App shows last loaded feed/products/health when offline
• Clear user messaging when data may be stale

References

• docs/01_CODEBASE_ARCHITECTURE_REVIEW.md: Offline support missing
• docs/02_FEATURES_REQUIREMENTS_BEST_PRACTICES.md: Non-functional req: cache 48h

[syed-reza98]

Closed as completed via PR #71.