One-tap sign-in for a fresh install, matching the pattern already shipped
in todo and crdt_sync_dart: openFirebaseWithGoogle() (google_platform.dart
abstracts the plugin's platform-channel dependence), isFirebaseConfigured()
that reads the stored session rather than trusting the account marker
alone (a marker with no live session is what made a synced phone read as
"not connected" for a full session), and storedAccount() for read-back
without the desktop-wrapper fallback that throws on Android.
Also generalizes crdt_sync_dart's ref pin to v0.10.0 and vendors the
override-guard script for the pre-commit hook already wired to it.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KdSp6Nbrs9xvsWkCMDN9a1
