[Frontend] Add transaction replacement and cancellation states to wallet UX

## Problem
Users can speed up, replace, or cancel a pending Stellar transaction, but a UI that only tracks the original hash may report the wrong final status or invite duplicate submissions.

## Scope
Model transaction lifecycle as a state machine with replacement/cancellation detection and clear finality messaging.

## Acceptance criteria
- Pending, submitted, confirmed, failed, replaced, cancelled, and unknown states are distinct.
- Polling stops when a terminal state is reached and resumes safely after reload.
- A replacement cannot be mistaken for a second user action.
- The UI links to the correct final transaction and preserves an audit-friendly history.
- Tests cover reload, timeout, replacement, cancellation, network failure, and duplicate clicks.

The issue names no files or test paths; start by locating the wallet transaction polling and original-hash tracking entry points. Add coverage for reload, timeout, replacement, cancellation, network failure, and duplicate clicks, and verify distinct terminal states, correct final-transaction links, safe polling, and audit history.

Written by the indexing model from the issue text.