feat(client): decode contract error codes into typed SDK errors

[DammyAji]

Summary

Parses Error(Contract, #N) from Stellar RPC simulation and submission failures and maps them to typed error subclasses, so callers can use instanceof checks instead of parsing XDR dumps.

What changed

• packages/client/src/errors.ts — Added 8 typed error subclasses (CircleNotFoundError, RoundNotFundedError, WrongRoundTagError, AlreadyClaimedError, InvalidProofError, RoundFullError, OverflowError, CircleCancelledError) matching the #[contracterror] enum in contracts/sharibo/src/lib.rs
• packages/client/src/decodeError.ts (new) — Regex-based parser that walks the error cause chain to extract Error(Contract, #N) patterns and instantiate the correct typed subclass
• packages/client/src/contract.ts — Added the missing withRetry helper for transient RPC errors; wrapped every call site (simulation + submission) so thrown errors are normalised via decodeContractError
• app/src/App.tsx — toUiError now uses instanceof-based lookup with human-friendly messages for each contract error; no XDR string matching
• docs/errors.md (new) — Shared error code table with codes, TypeScript class names, meanings, and sync guide
• packages/client/src/decodeError.test.ts (new) — 17 vitest tests using recorded real RPC error payloads

Acceptance criteria

• catch (e) { if (e instanceof AlreadyClaimedError) ... } works
• toUiError in the app contains no string matching on XDR
• All typed subclasses extend ContractError then ShariboError
• Original error preserved as cause on every subclass
• Unknown contract codes produce ContractError with the numeric code
• Non-contract RPC errors wrapped as RpcError
• 17 tests pass with recorded real RPC payloads

Closes #281

[DammyAji]

hi @Otaiki1 please kindly review and merge. Thank you.