refactor: enhance balance refresh logic in useStellarWallet and Walle…

[D240021]

fix(wallet): surface balance refresh errors on WalletScreen

Description

WalletScreen used to refresh the XLM balance without handling failure. A Horizon/network error was swallowed, so the user kept seeing a stale or zero balance with no way to tell the data was wrong. This change surfaces that failure, lets the user retry, and marks the last known balance as possibly out of date.

refreshBalance now returns true/false instead of swallowing the error. It still does not throw, so existing fire-and-forget call sites stay safe. stellar.getBalance() already distinguishes an unfunded account (NotFoundError → '0') from a network failure (the error is rethrown); that contract is unchanged.

Closes #84

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that breaks existing functionality)
• Documentation update
• Chore (deps, tooling, refactoring)

How Has This Been Tested?

• Unit tests added / updated
• Integration tests added / updated
• Manual testing on Android
• Manual testing on iOS

Unit tests cover:

• stellar.getBalance(): unfunded account → '0'; network failure → reject
• useStellarWallet.refreshBalance(): keeps the last known balance on failure; returns false; clears the error after a successful retry
• WalletScreen: shows Failed to refresh balance and May be out of date without replacing the last known balance; Retry and pull-to-refresh both re-fetch and clear the error on success

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented on complex or non-obvious code
• I have updated the documentation where needed
• My changes generate no new warnings or lint errors
• All existing and new tests pass

Screenshots

N/A — no visual assets attached. On refresh failure the wallet screen shows an error alert with a Retry button, the last known XLM amount, and a May be out of date warning. Pull-to-refresh is also available.

Additional Context

Completed from #84:

• refreshBalance reports success/failure to the UI without wiping the stored balance
• WalletScreen shows Failed to refresh balance, a retry control, and a stale-data indicator
• Tests cover error display and retry (button + pull-to-refresh)

Already in the repo (not changed here): src/services/stellar.ts already distinguishes NotFoundError from network errors. stellarBalance.test.ts still covers that contract.

Out of scope (per the issue): offline balance caching and background balance refresh.

I remain open to any corrections or feedback you may have.