[codex] Smooth social sign-in transitions

[jjoonleo]

Summary

• Keep Apple and Google sign-in buttons visible while the OnTime session is still being established, and disable repeat taps during that pending state.
• Move social sign-in orchestration into SignInMainScreen so Apple and Google share pending, cancellation, and failure behavior.
• Show the existing destructive/error-styled modal for social sign-in failures instead of a snackbar.
• Redirect authenticated first-install users directly to the notification allow route when notification permission is required, even if the alarm gate is still resolving.

Why

Provider authentication can complete before the OnTime backend session is established. That left a gap where the native/provider prompt disappeared but the app had not routed into the signed-in experience yet. On first install, the router could also detour through splash because unresolved alarm state was checked before the required notification gate.

Validation

• dart format on changed Dart files
• flutter test --reporter compact
• flutter analyze
• git diff --check