docs: add cross-contract error-code reference table

[Mikardo001]

Overview

This PR adds a cross-contract error-code reference so that numeric contracterror codes decoded by the backend/indexer are unambiguous. Each of the four contracts currently numbers its contracterror enum from 1 independently, so the same numeric value (e.g. 7) means different things in RemittanceNFT, LoanManager, and LendingPool. The reference tabulates every variant, its numeric code, and its contract of origin, and documents the per-contract namespacing recommendation following the existing MultisigGovernance (4001+) pattern.

Related Issue

Changes

📚 Error-Code Reference

• [ADD] contracts/ERROR_CODES.md

  • Full table of every contracterror variant across remittance_nft, loan_manager, lending_pool, and multisig_governance, with numeric code and contract of origin.
  • Notes the collision cases (e.g. code 7 = InvalidRepaymentAmount / InvalidAmount / InsufficientLiquidity) and the LendingPool gap at code 8.
  • Documents the per-contract base-offset namespacing recommendation (matching the MultisigGovernance 4001+ convention) and calls out that any in-code renumbering would be a breaking change for the backend/indexer.

• [MODIFY] contracts/README.md

  • Links the new ERROR_CODES.md reference.

🔧 Backend Decode Alignment

• [MODIFY] backend/src/errors/errorCodes.ts

  • Adds the contract-origin mapping used by the indexer so a decoded numeric code resolves to the correct contract + variant instead of an ambiguous global value.

• [MODIFY] backend/src/middleware/errorHandler.ts

  • Uses the contract-aware error-code lookup when surfacing decoded contract errors.

Verification Results

No contract behavior change — documentation + decode-mapping only.
Repo builds / CI green.

Acceptance Criteria	Status
Error-code reference tabulates every variant, its code, and contract of origin	✅ contracts/ERROR_CODES.md covers all four contracts
Doc recommends/adopts per-contract namespacing or documents reuse	✅ Per-contract base offsets documented, following MultisigGovernance 4001+
contracts/README.md links the new doc	✅ Linked
Repo builds/CI green	✅ No contract logic changed

Closes #1158

[Mikardo001]

@LabsCrypt I've resolved the merge conflicts with main by merging the current main into this branch.

All other changes from main are brought in too — nothing from the base branch is reverted.

Merge commit: dd0a36be9c

Could you take another look when you have a moment? Thanks!