core: implement offline-first with queue and sync

[EmeditWeb]

Problem

The app requires constant internet connectivity. In
emerging markets where StepFi targets learners,
internet connections are unreliable.

What To Build

1. offline-queue.ts for action queuing
2. offline-sync.ts for queue processing
3. Connectivity hook with NetInfo
4. Cache strategy with AsyncStorage TTL

Files To Touch

• src/offline/ (new module)
• hooks/useConnectivity.ts (new)
• services/api.ts
• app/_layout.tsx

Acceptance Criteria

• App loads cached data when offline
• Repayment queued when offline
• Queue processes when back online
• No crashes when network unavailable
• npx expo export --platform web passes

Mandatory Checks Before PR

• npx expo export --platform web passes
• Tested with airplane mode enabled
• PR references this issue

[BigElv]

I would love to work on this

[OG-wura]

hello maintainer. i would love to work on this

[grantfox-oss]

🦊 GrantFox — @OG-wura has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #12)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.