Clean up use_build_context_synchronously (batch, no behavior change)

Scope:\n- Dialogs/pages using BuildContext after await\n\nApproach:\n- Pre-fetch Navigator/ScaffoldMessenger\n- Check mounted after await\n- Use maybeOf for tolerance where needed\n\nPlan:\n- Small PRs (3–5 files each)\n- No behavior changes, only safety/consistency\n\nTracking:\n- Link related PRs to this issue.