feat: Optimistic UI for register/claim with rollback

[joelpeace48-cell]

Background & current state

On register/claim the UI waits on chain confirmation with little immediate feedback (TransactionStatus.jsx shows status, but state isn't optimistic). This feels slow.

Goal

Add optimistic UI for register/claim: reflect the expected state immediately on submit, then confirm or roll back on result.

Technical design

• On submit, optimistically update local state (e.g., "registered", balance change); on confirmation, reconcile with chain truth; on failure, roll back + show a mapped error (errorMapping.js).
• Guard against double-submit; show pending indicators.

Edge cases

• Tx fails after optimistic update → clean rollback to correct state.
• Slow confirmation → keep pending UI, allow cancel where possible.
• Concurrent actions → consistent reconciliation.
• Network/RPC error vs contract revert → distinct messaging.

Task breakdown

• Optimistic update + reconcile/rollback for register + claim.
• Double-submit guard + pending UI.
• Error-class-specific rollback messaging.
• E2E forcing a failure (assert rollback).

Acceptance criteria

• Successful actions feel instant; failed actions roll back to the correct state with a clear error.

Testing & verification

• E2E forcing tx failure → rollback; success path reconciliation.

Out of scope

• Backend changes.

Dependencies / related

• Uses errorMapping.js/TransactionStatus.jsx; pairs with NEW-108.

Difficulty: medium · Effort: M