[feature]: Single-screen login fast-path when password is the only enabled auth method

Is there an existing issue for this? • I have searched the existing issues Summary Summary When an instance has only email+password enabled (magic login off, no OAuth providers), the identifier-first flow — email screen → backend emailCheck → password screen — adds a roundtrip and a screen for a decision that has exactly one outcome. Render email + password on one screen in that case. Current behavior auth-root.tsx always starts at EAuthSteps.EMAIL, submits the email to the emailCheck endpoint (auth.service.ts), and only then routes to EAuthSteps.PASSWORD / UNIQUE_CODE and SIGN_IN / SIGN_UP. This branching exists to pick between password, magic-code, and OAuth. On an instance where password is the sole method there is nothing to branch on, yet users still pay: • an extra network roundtrip and a second screen/click on every login; • flaky password-manager autofill — the password field isn't in the DOM at page load, so Bitwarden/1Password can't do a st…