Fix: missing import, multi-sig persistence, Friendbot error handling, and exchange rate

[GoodnessJohn]

closes #215 — Add missing LanguageSelector import (frontend/src/App.jsx)

was rendered in JSX but never imported, causing a runtime crash. Added:

import { LanguageSelector } from './components/LanguageSelector';

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closes #217 — Persist pending multi-sig transactions to database (backend/src/services/multiSig.js,
backend/prisma/schema.prisma)

Pending multi-sig transactions were stored in a module-level Map, which is wiped on every server restart,
crash, or when running multiple instances.

• Added a PendingMultiSigTx Prisma model to schema.prisma
• Replaced all pendingTransactions.set/get calls in buildMultiSigTransaction, addSignature,
  submitMultiSigTransaction, getPendingTransactions, and getPendingTransaction with prisma.pendingMultiSigTx
  CRUD operations

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closes #221 — Check Friendbot response status (backend/src/services/stellar.js)

The Friendbot fetch call was fire-and-forget — a non-2xx response (rate limit, account already exists, network
error) was silently swallowed and the account was returned as if it were funded. Now checks response.ok and
throws a descriptive error on failure:

const friendbotRes = await fetch(https://friendbot.stellar.org?addr=${publicKey});
if (!friendbotRes.ok) {
throw new Error(Friendbot funding failed: ${friendbotRes.status} ${friendbotRes.statusText});
}

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closes #222 — Return 503 when exchange rate is unavailable (backend/src/routes/stellar.js)

When the orderbook had no asks, getRate returned null and the route responded { from, to, rate: null } with
HTTP 200, making it indistinguishable from a valid zero rate. The route handler now returns a 503 with a
descriptive message when rate === null.

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