Add Google OAuth sign-in via Supabase Auth on the login page

[Copilot]

This PR adds Google login support through Supabase Auth so users can authenticate without email/password. The login flow now supports both OAuth and existing credential-based sign-in with redirect preservation.

• Auth service: Google OAuth entrypoint

  • Added signInWithGoogle(redirectPath?: string) to AuthService.
  • Uses Supabase signInWithOAuth with provider: 'google'.
  • Reuses existing redirect URL construction to preserve post-login navigation.

• Login page: OAuth action + state handling

  • Added a Continue with Google action on the sign-in screen.
  • Wired button click to signInWithGoogle(...) using current redirectTo query param.
  • Reused existing submit/error state model so OAuth errors surface consistently.

• Login UI updates

  • Inserted OAuth-first layout with a lightweight separator before email/password fields.
  • Kept existing password login flow unchanged.

• Spec coverage for new flow

  • Extended login specs to verify:
    • latest redirectTo value is passed to Google sign-in
    • OAuth error message is surfaced to the page state

const { error } = await this.authClient.signInWithOAuth({
  provider: 'google',
  options: {
    redirectTo: this.buildRedirectUrl(redirectPath),
  },
});

[supabase]

This pull request has been ignored for the connected project cpibdotiiptojhawnpff because there are no changes detected in backend/supabase directory. You can change this behaviour in Project Integrations Settings ↗︎.

---

Preview Branches by Supabase.
Learn more about Supabase Branching ↗︎.