feat: map contract error codes 1-5 to human-readable messages in the SDK

[theo372001]

What changed

Adds describeContractError(code) to packages/client/src/errors.ts, covering the 5 stable error codes documented in contracts/sharibo/src/lib.rs (CircleNotFound, RoundNotFunded, WrongRoundTag, AlreadyClaimed, InvalidProof) — each returning a name, a user-facing sentence, and a hint. Adds parseContractErrorCode() to extract the numeric code out of a signAndSend() failure, and describeError() to tie both together with a raw-string fallback for anything unrecognized. Wires this into the app's claimAgain deliberate-replay demo so it now shows "AlreadyClaimed: ..." prose instead of the raw Error(Contract, #4).

Why

Closes #53

When the contract rejects a claim, users currently see Soroban's raw Error(Contract, #4) instead of something readable.

Implementation notes

• Failure shape: signAndSend()'s underlying @stellar/stellar-sdk (AssembledTransaction) throws a plain Error whose .message embeds Error(Contract, #<code>) — I traced this through the installed SDK's contract/assembled_transaction.js (simulationData getter → SimulationFailed with the RPC's raw diagnostic string) and contract/utils.js, which defines its own contractErrorPattern = /Error\(Contract, #(\d+)\)/ for exactly this purpose. This matches the existing raw-string checks already in this repo at scripts/e2e.ts's replay path (message.includes("Error(Contract, #4)")) and scripts/smoke.ts (msg.includes("Error(Contract, #1)")). I was not able to run scripts/e2e.ts live — it requires a deployed testnet contract/admin secret I don't have, and the script currently has unrelated pre-existing syntax errors around its funding/proof-generation steps (duplicated blocks, mismatched braces) that predate this change and are out of scope here. parseContractErrorCode()'s regex is a superset of the SDK's own pattern and is covered by unit tests using both a realistic multi-line simulation-failure message and the exact minimal strings already used in e2e.ts/smoke.ts.
• getErrorMessage in app/src/App.tsx: this function was already called in claimAgain's catch block and in the Freighter fundMember path, but was never defined anywhere in the file (a pre-existing bug — the app didn't compile). I added it as a small wrapper around the new describeError(), mirroring the existing toUiError helper's Friendbot special-case.
• index.ts export: packages/client/src/index.ts didn't re-export errors.ts even though App.tsx and scripts/e2e.ts already import ContractError/RpcError from @sharibo/client — also pre-existing and fixed here since it's required for this feature to be importable at all.
• This package (packages/client) has substantial pre-existing, unrelated breakage on main (missing exports in tree.ts/prove.ts/config.ts, an undefined withRetry in contract.ts, a duplicate MerkleTree identifier in tree.test.ts, etc. — confirmed via tsc --noEmit before and after this change, identical error list both times). I did not touch any of that; it's out of scope for Map on-chain error codes #1–#5 to human-readable messages in the SDK #53.

Testing

• packages/client/src/errors.test.ts (new, 12 cases covering all 5 known codes, 4 unknown/out-of-scope codes 0/6/7/8/999, the exact signAndSend-shaped failure string, the minimal e2e.ts/smoke.ts strings, a ContractError instance, non-contract errors, and non-Error throws) — run via npx tsx --test src/errors.test.ts (the package's own vitest run doesn't pick up any of this package's node:test-style test files, including the pre-existing ones, due to a pre-existing test-runner mismatch unrelated to this change) — 12/12 passing.
• npx tsc --noEmit in packages/client and app — my changes introduce zero new type errors versus the pre-existing baseline (verified by diffing output before/after via git stash).
• Circuit tests (not touched)
• Contract tests (not touched)
• E2E against testnet — not run; no deployed contract/testnet credentials available in this environment, and the script has pre-existing unrelated syntax errors (see above)
• App tested manually (cd app && npm run dev) — not run end-to-end; no live testnet contract available to exercise the actual replay demo in the browser. Unit-verified the exact string transformation instead (Error(Contract, #4) → "AlreadyClaimed: this proof's nullifier was already used; ...") via the describeError test cases above.
• Docs-only: N/A (code changed)

Screenshots

Not applicable — no visual/layout change, only the text shown in the existing rejection banner.