Restructure src/ and tests/ from package-by-type to package-by-domain

All classes are now organised by domain (Availability, Booking, Auth).
Each domain package contains its value object, repository, admin controller,
REST endpoint, and any shortcode pages under a matching sub-namespace.
Cross-cutting wiring (Plugin, AdminMenu, RestRegistrar, ShortcodeRegistrar,
Schema) lives at src/ root. Tests mirror the domain structure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Auth/LoginPage.php

@@ -0,0 +1,49 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+class LoginPage {
+
+	/**
+	 * Renders the student login shortcode output.
+	 *
+	 * @param array<string, string> $atts Shortcode attributes (unused — reserved for future options).
+	 */
+	public function render( array $atts ): string { // phpcs:ignore Generic.CodeAnalysis.UnusedFunctionParameter.Found
+		if ( is_user_logged_in() ) {
+			$redirect = esc_url( (string) get_permalink() );
+			return sprintf(
+				'<p>%s <a href="%s">%s</a>.</p>',
+				esc_html__( 'You are already logged in.', 'unsupervised-schedular' ),
+				$redirect,
+				esc_html__( 'View available lessons', 'unsupervised-schedular' )
+			);
+		}
+
+		$error    = '';
+		$redirect = sanitize_url( (string) get_permalink() );
+
+		if ( isset( $_POST['us_login'] ) && check_admin_referer( 'us_student_login' ) ) {
+			$credentials = [
+				'user_login'    => sanitize_user( wp_unslash( $_POST['log'] ?? '' ) ),
+				// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- passwords must not be sanitized.
+				'user_password' => wp_unslash( $_POST['pwd'] ?? '' ),
+				'remember'      => isset( $_POST['rememberme'] ),
+			];
+
+			$user = wp_signon( $credentials, false );
+
+			if ( is_wp_error( $user ) ) {
+				$error = esc_html__( 'Invalid username or password.', 'unsupervised-schedular' );
+			} else {
+				wp_safe_redirect( $redirect );
+				exit;
+			}
+		}
+
+		ob_start();
+		include USC_PLUGIN_DIR . 'templates/frontend/login-page.php';
+		return (string) ob_get_clean();
+	}
+}