[Banx17]

Summary

Closes #2143.

Adds frontend/src/app/utils/soroban.networkMismatch.test.ts, a regression suite for the two XDR builders in frontend/src/app/utils/soroban.ts (buildUnsignedLoanRequestXdr and buildUnsignedRepaymentXdr). Every case runs against both builders, with the RPC server stubbed so the suite stays offline and deterministic.

The contract the suite pins is fail closed: the builder must reject (so transactionErrors/mapTransactionError can keep classifying the failure) and must never return an unsigned XDR that was not simulated on the wallet's network.

Acceptance criteria coverage

Area	Tests
Network mismatch	the returned XDR is bound to the network the wallet will sign for: its signature base carries that network's id (derived from the SDK, not a re-implemented hash) and not the other network's; the same invocation hashes differently on TESTNET vs PUBLIC; a signature verifies only on the network the XDR was built for (public-key verification against the cross-network hash fails); an explicit networkPassphrase beats NEXT_PUBLIC_STELLAR_NETWORK_PASSPHRASE; the shared DEFAULT_NETWORK_PASSPHRASE is used when neither is given; a mismatch reported by the RPC propagates as a rejection with no XDR returned.
Simulation timeout	a timed-out simulation rejects after exactly one attempt and never serializes the unprepared transaction; a still-pending simulation keeps the promise unsettled (and resolves normally once released), proving the builder genuinely awaits prepareTransaction instead of racing it.
Malformed RPC responses	a malformed getAccount payload rejects before simulation; a failed account lookup rejects before simulation; a null simulation response and a non-transaction simulation payload are rejected instead of serialized; a malformed JSON-RPC error object is propagated by identity.
Authorization	invocations ship with no pre-filled auth entries (3 contract args, the wallet signs), and HostError: Error(Auth, InvalidAction) raised during simulation propagates untouched.
Input validation	NaN / Infinity / 1e21 amounts and unparseable contract ids are rejected before simulating; the u32 term and u64 loan-id boundaries are accepted; a fractional loan id throws SyntaxError instead of being truncated, and a negative loan id is rejected as unsigned.

Changes

• frontend/src/app/utils/soroban.networkMismatch.test.ts — new suite, 42 assertions-level tests (21 per builder).
• .github/workflows/ci.yml — the existing "Frontend regression tests" step now also runs this suite, so a network/simulation regression fails CI instead of only local runs.

Test plan

cd frontend
npx jest src/app/utils/soroban.networkMismatch.test.ts --runInBand   # 42 passed
npx jest src/app/hooks/useLoanStream.test.tsx \
         src/app/components/providers/SessionExpiryHandler.test.tsx \
         src/app/utils/soroban.networkMismatch.test.ts --runInBand   # 55 passed (CI step)
npx eslint src/app/utils/soroban.networkMismatch.test.ts             # clean
npx prettier --check src/app/utils/soroban.networkMismatch.test.ts   # clean

Notes

• No production code changed; the suite is additive and asserts the behaviour soroban.ts already has, so it fails if the builders regress (dropped networkPassphrase, serializing before/without simulation, swallowing RPC errors).
• Pre-existing and unrelated to this PR: frontend/src/app/components/providers/WalletProvider.tsx:249 references NETWORK_PASSPHRASES without importing it, so soroban.networkSigner.test.tsx fails with ReferenceError: NETWORK_PASSPHRASES is not defined on main. Left untouched to keep this PR scoped; happy to fix it separately if you want.