test: widget tests for the router redirect and consent gate

## 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