Fix payment confirmation, modal focus, API URL, and empty error toasts

[Valreb001]

Closes #926
Closes #925
Closes #924
Closes #923

Summary

7b159f2: Wait for ledger confirmation before showing payment success (#926)

• Added waitForLedgerConfirmation in new utils/ledgerConfirmation.ts; payInvoice in utils/soroban.ts now polls getTransaction until SUCCESS before returning success.
• A FAILED or timed-out transaction returns an error result instead of a premature success message.

5d8b400: Restore keyboard focus when the payment modal closes (#925)

• useFocusTrap cleanup now retries focusing the trigger element after the DOM settles if the first attempt did not take.

25549ab: Resolve API base URL per request to avoid stale config (#924)

• utils/api.ts replaces the module-level API_BASE constant with getApiBase(), read on every request.

d3f7fca: Fall back to a default message for empty API error notifications (#923)

• ToastProvider.addToast in Toast.tsx substitutes a default message when the message is empty or not a string.
• request in utils/api.ts only uses body.error when it is a non-empty string, otherwise the status-based message.

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