F3: rename SparkboardOnboardingHook.afterFirstLogin(OnboardingContext)
→ onFirstSignIn(AuthenticatedUser) across Architecture, Integration-Guide,
Sprint-1-Plan-Part-2, Sprint-1-Testplan, Home.
plate-auth's spelling wins; aligns with plate-auth Architecture +
Sprint-0-Plan SPI signature.
F4: Sprint-1-Testplan UT-06 test class moved from
de.plate.sparkboard.auth.SparkboardOnboardingHookTest →
de.plate.sparkboard.onboarding.SparkboardOnboardingHookTest.
Matches the package declared in Sprint-1-Plan-Part-2 +
Integration-Guide §2.
Home.md hook link text also updated to onFirstSignIn().
Plan-Review.md intentionally left untouched (historical record).