fix(ui): clear accepted leave navigation targets

[rasitakyol]

What?

• Consume a cancelled navigation target before starting the accepted route transition.
• Extend the admin E2E regression through a subsequent Create New navigation.

Why?

The accepted URL remained in a ref after the first transition. If the effect ran again, it could push the stale list URL and immediately undo the user's next navigation.

How?

• Snapshot and clear the ref before invoking onAccept and router.push.
• Verify the full dirty form, Leave anyway, list, and Create New flow in Chromium.
• Validated with Node 24.15.0 and pnpm 11.9.0: the focused E2E passed 1/1, pnpm build:ui --force passed, and Prettier, source ESLint, and git diff --check passed.

Fixes #17233

[rasitakyol]

CI note: the current failing job is E2E - lexical__collections__LexicalHeadingFeature [next]. It times out waiting for [data-lexical-editor="true"] in test/lexical/collections/LexicalHeadingFeature/e2e.spec.ts, before the navigation flow changed by this PR runs.

This PR only changes usePreventLeave.tsx and the general admin E2E regression. The focused Chromium regression passes locally (1/1), and pnpm build:ui --force passes. All Green is failing only as the aggregate of that Lexical job.

Could a maintainer rerun the failed Actions job? I do not have permission to rerun workflows in the upstream repository.