Show a sign-in link instead of the form after email confirmation

Closes #67

When a student lands on the registration page from the confirmation
email (?us_confirmed=1), replace the registration form with the
confirmation message and a "Sign in to your account" link — the form
is useless at that point and re-submitting would only produce an
"account already exists" error. A confirmed-but-unapproved student can
already log in (the pending gate only withholds booking), so signing in
is the natural next step.

The link target follows the booking block's pattern: a loginPageId
block attribute (page picker in the editor sidebar) or login_page_id
shortcode attribute, falling back to wp_login_url(). The expired-link
notice keeps the form as before.

Co-Authored-By: Claude Fable 5 <[email protected]>

src/Auth/RegistrationPage.php

@@ -31,9 +31,10 @@ class RegistrationPage {
 	/**
 	 * Renders the student registration shortcode output.
 	 *
-	 * @param array<int|string, mixed> $atts Shortcode attributes (unused — reserved for future options).
+	 * @param array<int|string, mixed> $atts Block attributes (`loginPageId`) or
+	 *                                       shortcode attributes (`login_page_id`).
 	 */
-	public function render( array $atts ): string { // phpcs:ignore Generic.CodeAnalysis.UnusedFunctionParameter.Found
+	public function render( array $atts ): string {
 		if ( is_user_logged_in() ) {
 			return '<p>' . esc_html__( 'You already have an account and are logged in.', 'unsupervised-schedular' ) . '</p>';
 		}
@@ -60,6 +61,9 @@ class RegistrationPage {
 		// phpcs:ignore WordPress.Security.NonceVerification.Recommended -- read-only display flag, not a state change.
 		$confirmResult = sanitize_key( Val::string( wp_unslash( $_GET['us_confirmed'] ?? '' ) ) );
 
+		// Where the post-confirmation prompt sends students to sign in.
+		$loginUrl = $this->loginUrl( Val::int( $atts['loginPageId'] ?? $atts['login_page_id'] ?? 0 ) );
+
 		$policyForms = $this->signupPolicies();
 		$canRegister = $open || ( null !== $invite && $invite->isAcceptable( current_time( 'mysql' ) ) );
 
@@ -179,6 +183,23 @@ class RegistrationPage {
 		return self::RESULT_CONFIRM;
 	}
 
+	/**
+	 * URL the post-confirmation sign-in link points to: the chosen login page
+	 * when one is configured (and still exists), otherwise the WordPress login
+	 * screen.
+	 */
+	private function loginUrl( int $loginPageId ): string {
+		if ( $loginPageId > 0 ) {
+			$url = get_permalink( $loginPageId );
+
+			if ( is_string( $url ) ) {
+				return $url;
+			}
+		}
+
+		return wp_login_url();
+	}
+
 	/**
 	 * Build the email-confirmation URL for a raw token: the configured
 	 * registration page (falling back to the home page) with `?us_confirm=`.

src/BlockRegistrar.php

@@ -104,7 +104,12 @@ class BlockRegistrar {
 			],
 			'us-scheduler/student-register' => [
 				'render'     => [ $this, 'renderRegistration' ],
-				'attributes' => [],
+				'attributes' => [
+					'loginPageId' => [
+						'type'    => 'number',
+						'default' => 0,
+					],
+				],
 			],
 			'us-scheduler/group-classes'    => [
 				'render'     => [ $this, 'renderGroupClasses' ],