Same pattern as todo, diet_guard_app, and workout_app: openFirebaseWithGoogle()
(google_platform.dart abstracts the plugin's platform-channel dependence),
isFirebaseConfigured() reading the stored session rather than the account
marker alone, and the settings screen offering "Sign in with Google" ahead
of the password form.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KdSp6Nbrs9xvsWkCMDN9a1
