feat(mobile): pull-to-refresh + polling on the dashboard

[kaylachi]

Summary

Keeps the mobile dashboard fresh both ways (backlog #34 / #462): an explicit pull gesture and quiet background polling of balance + activity.

Added / changed

• lib/activity.ts — fetchDashboardData(publicKey) loads the account's native XLM balance and its 20 most recent payments from Horizon. The pure mapPayments turns Horizon payment/create_account records into direction-aware activity rows (received-from / sent-to), dropping non-transfer ops. An unfunded account (Horizon 404) reads as a zero balance with no activity, not an error.
• hooks/usePolling.ts — a ref-backed setInterval (the native stand-in for the web dashboard's service-worker polling) that re-fires every POLL_INTERVAL_MS and never restarts when the callback identity changes; always cleared on unmount / when disabled.
• app/index.tsx — the placeholder home route is now the dashboard: a balance card + activity feed inside a ScrollView with RefreshControl (pull-to-refresh) and background polling while a wallet is present. Distinct loading / no-wallet / empty / error states.

Testing

• tsc --noEmit ✅
• jest lib/__tests__/activity.test.ts ✅ (6/6 — direction, counterparty, non-native asset code, create_account, non-transfer filtering, timestamp conversion)

Closes #462

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[Miracle656]

Thanks! This needs a rebase — main has moved a lot on the dashboard since this branch was cut (app/(tabs)/dashboard.tsx and the activity/polling stores landed in early August). Could you rebase onto main and confirm what pull-to-refresh/polling here is still net-new versus what already ships? Happy to re-review once it applies cleanly.

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[kaylachi]

Hi! I've rebased this branch onto main. The background polling of the activity feed is indeed already handled on main via useInitActivityFeed. I've reconciled this PR by adding background polling for the XLM balance itself and wiring the dashboard gesture to pull-to-refresh (which refreshes both the balance and the activity feed). Ready for another look!