test: widget tests for the router redirect and consent gate

[Zeroupper]

Problem

(verified) The redirect chain (lib/carp_study_app.dart:35-64) and consent gating (lib/ui/pages/home_page.dart:41-70) encode the app's entire onboarding correctness and are untested — these are the paths the boundary EPIC and #599 will rework.

Proposed fix

Widget tests pumping MaterialApp.router with a faked bloc/coordinator: unauthenticated → /login; no study → /invitations; consent pending → consent page; configured → study page. Write them before the HomePage refactor as a safety net.

Acceptance criteria

• Four redirect scenarios green pre- and post-refactor

Effort: M · Priority: P1 · Change risk: None