feat(explorer): add Privy card funding

> [!IMPORTANT]
> Current execution is tracked by #436. This issue owns the Privy card-funding flow in PR #418, after #377 / PR #417. Browser-audited recovery-marker corrections are owned separately by #445 / PR #431. Historical references below are retained for attribution.

Parent tracker: #393
Depends on: #396 and the #381 replacement PR

## Goal

Let an authenticated Privy user buy a supported source asset by card and continue into the existing direct Filecoin Pay funding flow.

## Scope

- Add the Privy card/onramp launch panel and purchase hook.
- Preserve the intended Filecoin Pay account and destination amount.
- After purchase, select the actual landed chain/token and launch the existing payment flow.
- Surface cancellation, provider failure, unsupported landing asset, and delayed balance states.

## Non-goals

- Production Privy plan provisioning or billing approval; #382 owns that work.
- A separate Squid executor.
- Silent automatic signing after a purchase.

## Acceptance criteria

- [ ] Successful purchase resumes the intended funding destination.
- [ ] Cancellation and provider errors are recoverable.
- [ ] Landed asset selection is verified rather than assumed.
- [ ] Account or wallet changes invalidate stale continuation state.
- [ ] Focused automated tests and documented sandbox/preview evidence pass.
- [ ] Repository-required checks pass.

Performance classification: not performance-relevant; external provider latency is observational and the console must remain responsive.

Start with the current execution tracked in #436 and the Privy flow in PR #418; review the parent tracker #393 and dependencies #396 and #381. Verify the purchase continuation, landed asset selection, cancellation and provider-error recovery, stale account or wallet state, focused tests, sandbox or preview evidence, and repository-required checks.

Written by the indexing model from the issue text.