feat(sdk): surface typed contract error taxonomy end-to-end (#404)

[Anadudev]

Closes #404

Summary of Changes

• Typed ContractError Class: Converted ContractError into a typed, instanceof-checkable Error class carrying code: number | null, friendly: string, and raw: string.
• SDK Error Taxonomy Integration: Exported ContractError, the canonical PROOF_REGISTRY_ERRORS code-to-message lookup table, and isRetryableContractError directly from @stellarcred/sdk and attached them to the StellarCred namespace export.
• Contract Submit Flow Alignment: Updated sendAndConfirm and submitProofs in frontend/lib/contracts.ts to directly parse and throw ContractError instances upon contract execution/simulation failure rather than plain generic Error instances.
• UI Branching & Retry Logic: Updated frontend/app/holder/page.tsx proof submission catch handlers to branch on e instanceof ContractError and use isRetryableContractError(error) to determine whether retry actions are shown or if a terminal failure state should be displayed.
• Test Coverage: Added comprehensive tests in frontend/lib/__tests__/contracts.test.ts and frontend/packages/sdk/src/index.test.ts verifying typed error mapping, instanceof compatibility, namespace exports, and retryability branching.

Acceptance Criteria Verified

• Contract panics surface as typed SDK ContractError instances with error codes and friendly messages.
• UI branches programmatically on ContractError and evaluates retryable vs terminal states (isRetryableContractError).
• PROOF_REGISTRY_ERRORS serves as the single source of truth across the stack.

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