feat: implement mobile transaction cancellation handling and related …

[Elizabeth-dev270]

Closes #199

✅ Cancellation handling has been wired up cleanly

I updated the payment/signing flow so an intentional cancel is kept separate from a real failure:

• In signerStore.ts, cancelSigning() now moves the state to cancelled and safely clears currentReview, lastResult, and error.
• In useSignerHandoff.ts, the pre-signing and pre-submission cancellation checkpoints now return a cancellation outcome instead of pushing the flow into a failed state.
• In review-transaction.tsx, the cancelled UI remains a neutral “Cancelled” state and routes the user back safely with no failure messaging.

Result

This satisfies the key behavior requested:

• Cancellation is represented separately from failure.
• Users can cancel before signing/submission.
• Cancelled flows return to a safe screen.
• No failure toast/error state is shown for intentional cancellation.
• Store state is cleared safely after cancellation.

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Verification

I reproduced the regression first with the targeted Jest test:

• Command run: npm test -- --runInBand __tests__/signerStore.cancelled.test.ts
• Evidence: it failed before the fix because the cancelled path still left error populated.

I then validated the edited files with static diagnostics, and signerStore.ts, useSignerHandoff.ts, and review-transaction.tsx report no file-level errors.

A fresh terminal Jest rerun could not be completed in this session because the command execution was skipped by the environment, so the code-level change is in place and statically clean, but the final runtime test confirmation remains to be executed in a normal terminal session.

Made changes.