Send flow has no optimistic UI or rollback once payments become real

[ndii-dev]

Currently sendPayment's fake success returns instantly-ish (no real network latency, no real failure modes), so the UI doesn't need to handle a submitted-but-not-yet-confirmed state. Once real Horizon submission lands (see issue above), the send flow needs an optimistic "pending" state immediately on submit, with rollback/error UI if the real submission fails after the optimistic update already rendered.

Definition of done:

• Optimistic transaction entry appended to the UI immediately on submit
• Rollback (removal or error-state marking) if the real submission subsequently fails
• No duplicate entry once the real confirmed transaction syncs in

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This is not a starter-issue. The Definition of done above is the full
acceptance criteria, not a subset to sample from — a PR that addresses part
of it is an unfinished issue, not a smaller one. Your PR must include, in
the PR description itself:

• Root cause / design-decision rationale in your own words — not a restatement of this issue
• Every Definition of done bullet above addressed explicitly, with a one-line note on how
• Evidence the code actually runs: pasted test/build output, a screen recording or before/after screenshots for UI changes, or real (non-mocked) logs for network/contract-facing work
• New or updated tests included and shown passing (paste the output)
• Any adjacent/related behavior this issue calls out re-verified, not assumed unaffected

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[iamb8t0-oss]

Hi i would like to work on this issue

[grantfox-oss]

🦊 GrantFox — @iamb8t0-oss has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #91)
2. Your PR will be reviewed by the Orbit-Wal maintainers

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