feat(mobile): balance card with Lens fiat value

[Lazyartist1]

Summary

Adds the dashboard's primary balance card (backlog item 30) — native XLM plus its fiat value from the Lens price oracle — the most-looked-at element in the product, so it loads fast and degrades gracefully.

Changes

• lib/fetchPrice.ts — native port of the web wallet's oracle client (frontend/wallet/lib/fetchPrice.ts), reading EXPO_PUBLIC_LENS_URL. Best-effort: every failure (402 x402 gating, 404, timeout, malformed body) collapses to null. Adds pure usdValue/formatUsd helpers.
• components/BalanceCard.tsx — self-loading card. Balance comes from Horizon (unfunded accounts read as 0, not an error) and is authoritative; the fiat line shows an em dash whenever the price feed is unavailable, so a slow/gated oracle never blocks the card. Loading, no-wallet, and error states included.
• Mounts the card on the home route (app/index.tsx).

Acceptance

• Shows XLM balance + USD via Lens
• Handles price-fetch failure (fiat degrades to —, balance still renders)

Testing

• tsc --noEmit clean
• jest lib/__tests__/fetchPrice.test.ts — 10/10 passing (USDC pin, success, non-OK/402, missing-price, network-error → null; usdValue + formatUsd)

Closes #458

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[Miracle656]

Thanks — the BalanceCard + fetchPrice additions look useful, but this needs a rebase: it's conflicting with main on app/index.tsx, which has changed since this branch was cut. Please rebase onto current main and re-wire the balance card into the current index screen. One note: the Lens fiat value depends on the Lens price service, which is currently down (503) — make sure the card degrades gracefully (shows the token balance without fiat) when Lens is unreachable. Then I'll re-review.

[Lazyartist1]

Hi! I've rebased this branch onto main, resolved the conflicts in app/index.tsx, and wired the BalanceCard component directly into the dashboard tab screen (app/(tabs)/dashboard.tsx) in place of the static placeholder balance hero. I've also verified that the card degrades gracefully by showing an em dash (—) for the fiat value when the Lens oracle service is down or unreachable. Ready for review!

[Miracle656]

Thanks for the rebase — moving the balance card into (tabs)/dashboard.tsx is the right call. Unfortunately it's conflicting again through no fault of yours: #587 (pull-to-refresh) landed on dashboard.tsx right after you pushed. One more rebase onto current main and resolving dashboard.tsx should clear it (your change is only ~7 lines, so it should be quick). While you're there, please double-check the card degrades gracefully when the Lens price service is unreachable (it's currently returning 503) — show the token balance without the fiat value rather than erroring. Then I'll merge.

[Lazyartist1]

Hi! I've rebased this branch onto the latest main (which now includes PR #587) and resolved the conflict in app/(tabs)/dashboard.tsx. The BalanceCard is successfully wired into the new dashboard screen alongside the pull-to-refresh and polling logic. Ready for merge!

[Lazyartist1]

Hi! I've updated the PR with a new unit test verifying that non-native assets are correctly formatted and encoded as CODE:ISSUER in the Lens price oracle requests, and verified that both fetchPrice and formatUsd/usdValue handle these assets generically. Ready for review!