feat: unify admin identity — SAAS_ADMIN_USER is the email in SaaS mode

In SaaS mode, SAAS_ADMIN_USER must be an email address. It's used as
both the Logto username and primaryEmail. No separate SAAS_ADMIN_EMAIL.
Installer enforces email format in SaaS mode (moved deployment mode
question before admin credentials), accepts any username in standalone.
Sign-in form label changed to "Login".

Removes SAAS_ADMIN_EMAIL from bootstrap, compose template, installers,
and all documentation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/sign-in/src/SignInPage.tsx

@@ -186,7 +186,7 @@ export function SignInPage() {
           {/* --- Sign-in form --- */}
           {mode === 'signIn' && (
             <form className={styles.fields} onSubmit={handleSignIn} aria-label="Sign in" noValidate>
-              <FormField label="Email or username" htmlFor="login-identifier">
+              <FormField label="Login" htmlFor="login-identifier">
                 <Input
                   id="login-identifier"
                   type="email"