fix(mobile): let users sign out after leaving all communities (#8993)

## Summary
Fixes a dead-end in signed-in mobile onboarding: after leaving every
community, Back kept returning to the same Add a community screen
without a way to end the account session.

Show “Signed in as [email]” on that screen. Back asks “Sign out of
BuilderLab?” with the email; Cancel keeps the session, and Sign out
returns to the welcome screen while retaining saved Buzz identities.
Nested Back and Add community from an existing session keep their
navigation behavior.

### Related issue
None found in the existing issue and PR search for onboarding sign-out.

### Testing
The new widget journey failed before the change and passes through the
real onboarding widgets, account client, storage and flow with external
HTTP/browser boundaries mocked. It covers nested Back, Cancel, system
Back, durable session removal, retained identity/backup, and starting
email sign-in again. The existing hosted onboarding journey also covers
accounts without an email.

On iPhone 18 Pro simulator (iOS 27), rendered the production onboarding
screen and native confirmation using a disposable account fixture. Live
browser account selection has not been verified; this clears the app
session, not website cookies.

To reproduce on an auth-enabled mobile build: sign in, leave all
communities, check the displayed email, tap Back, and choose Cancel. Tap
Back again and choose Sign out. The yellow welcome screen should appear,
with email sign-in available.

## Screenshots

Before, from the original reported phone screen:

<img
src="https://raw.githubusercontent.com/block/buzz/07440b3b10c3f48a6fd19c923f2ee5ba66d574f0/pr-8993--before.png"
alt="Before: account state not shown" width="300">

After:

Signed-in landing and native sign-out confirmation, using a disposable
account:

<img
src="https://raw.githubusercontent.com/block/buzz/a4e18c85bc3a19d971b08ff595ba6b081c67e20f/pr-8993--landing.png"
alt="Signed-in landing" width="300"> <img
src="https://raw.githubusercontent.com/block/buzz/a4e18c85bc3a19d971b08ff595ba6b081c67e20f/pr-8993--confirmation.png"
alt="Sign-out confirmation" width="300">

---------

Signed-off-by: Aaron <f425b86c3b780d43f8d9cf1802429d04dd985bf92eac44fca9468b5ad3887c5e@buzz.block.builderlab.xyz>
Signed-off-by: John Tennant <jtennant@squareup.com>
Co-authored-by: Aaron <f425b86c3b780d43f8d9cf1802429d04dd985bf92eac44fca9468b5ad3887c5e@buzz.block.builderlab.xyz>

.github/workflows/_ci-clients.yml

@@ -154,6 +154,7 @@ jobs:
           test/shared/hosted/relay_auth_isolation_test.dart
           test/features/onboarding/hosted_app_gate_environment_test.dart
           test/features/onboarding/hosted_journeys_test.dart
+          test/features/onboarding/onboarding_sign_out_test.dart
           test/features/onboarding/hosted_communities_test.dart
           test/features/pairing/pairing_hosted_handoff_test.dart
           test/shared/agents/remote_agents_test.dart