Splits the settings screen: Nutrition (daily kcal goal) and Notifications
(battery exemption) stay in place, Firebase+GitHub content is replaced by
a Sync settings link using the shared sync_settings_ui package. GitHub
stays app-local as its own GitHubMirrorScreen (device-flow connect,
_ClientIdSetupDialog, the wrapperManagedToken desktop-wrapper path) since
connecting it also triggers a real cross-device sync via runSync(), not
just a settings save.
BackupSlot left null: diet-guard has no local export/import feature.
553/553 tests passing, 100% coverage on settings_screen.dart, flutter
analyze clean.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KdSp6Nbrs9xvsWkCMDN9a1
