refactor: unify contract-error normalization into errors.ts

[Riel101]

Close #491

PR Description

Summary

Unify contract and blockchain error normalization into a single backend error layer so on-chain failures are classified consistently and retryability is preserved.

What changed

• Centralized blockchain error classification inside errors.ts
• Removed duplicate local contract normalization logic from contracts.ts
• Updated normalizeBackendError() so RPC timeout, rate-limit, and other contract errors are classified consistently and existing BackendError retryable behavior is preserved
• Added focused unit tests in errors.test.ts covering timeout, rate-limit, and fallback classification behavior
• Updated backend-error-codes.md to document the unified error normalization path

Validation

• Ran pnpm --dir /workspaces/Commitlabs-Frontend exec vitest run src/lib/backend/errors.test.ts
• Result: 32 passed

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