Pairs with website feat/sso-trusted-device, which makes "trust this device" work
for SSO sign-ins. Two things reach this device when the user ticks the box:
1. The rg_trust COOKIE in the Custom Tab. Custom Tabs share the system
browser's cookie jar, so that alone makes the next SSO sign-in skip the
TOTP step — no app change needed for that half.
2. A trustToken in the /auth/mobile/sso/exchange response, which is what this
commit stores. That covers the app's NATIVE password login on the same
device, which reads the token back out of TrustTokenStore and replays it as
X-Trust-Token.
MobileTokenResponse already carried trustToken (the native login path has always
persisted it) — SsoAuthManager simply dropped it on the floor. Save it scoped to
the signed-in username, exactly like AuthRepository.login does, so it is never
replayed for a different account on a shared device; and save it before
onSignedIn so a process death mid-callback can't lose it.
Tests: 2 new cases in SsoAuthManagerTest (token persisted + scoped to its owner;
absent token leaves the store untouched), with an in-memory FakeTrustTokenStore
matching the file's existing fake style. Full unit suite green: 266 tests.
Co-Authored-By: Claude <noreply@anthropic.com>
