Show instructor real name or nickname in group-class views, not the login

Add Auth\UserName::format(), which prefers a user's first + last name, then
their nickname, avoiding display_name (which can be the login/username).
Route the instructor name through it in both the front-end offerings response
(instructor_name) and the back-end group-class summary and details views.

Tests: composer test (513), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Auth/UserName.php

@@ -0,0 +1,35 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+/**
+ * Resolves a person's public-facing name for display. Prefers their real name
+ * (first + last), then their nickname — deliberately avoiding the account's
+ * login/username, which `display_name` can otherwise expose.
+ */
+class UserName {
+
+	/**
+	 * The display name for a user: "First Last" when a real name is set,
+	 * otherwise the WordPress nickname. Falls back to the numeric id (or an empty
+	 * string when none is given) when the user cannot be loaded or has no name.
+	 */
+	public static function format( ?\WP_User $user, int $fallbackId = 0 ): string {
+		if ( ! $user instanceof \WP_User ) {
+			return $fallbackId > 0 ? (string) $fallbackId : '';
+		}
+
+		$full = trim( $user->first_name . ' ' . $user->last_name );
+		if ( '' !== $full ) {
+			return $full;
+		}
+
+		$nickname = trim( $user->nickname );
+		if ( '' !== $nickname ) {
+			return $nickname;
+		}
+
+		return $fallbackId > 0 ? (string) $fallbackId : '';
+	}
+}