Add link-target and auto-redirect options to booking/login blocks

The booking block gains a loginPageId attribute choosing which page its
logged-out "log in to book a lesson" link points to (default remains the
WordPress login screen), and the student-login block gains a
bookingPageId attribute controlling the logged-in "View available
lessons" link and the post-login redirect target (default remains the
current page). Both blocks also gain an autoRedirect toggle, off by
default, that sends the visitor straight to the target page; block
rendering starts after output, so the redirect runs on
template_redirect by parsing the queried page's content for the block,
with a self-target guard against redirect loops. The link targets are
also available to the shortcodes as login_page_id/booking_page_id.

Also fixes a pre-existing fatal: WordPress passes an empty string (not
an array) to shortcode callbacks when a shortcode is used without
attributes, so bare [us_booking] etc. threw a TypeError against the
strictly-typed render(array $atts) methods. ShortcodeRegistrar now
wraps each callback to normalize non-array attribute values.

Closes #51

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

src/ShortcodeRegistrar.php

@@ -19,14 +19,26 @@ class ShortcodeRegistrar {
 	) {}
 
 	public function register(): void {
-		add_shortcode( 'us_booking', [ $this->bookingPage, 'render' ] );
-		add_shortcode( 'us_student_login', [ $this->loginPage, 'render' ] );
-		add_shortcode( 'us_student_register', [ $this->registrationPage, 'render' ] );
-		add_shortcode( 'us_group_classes', [ $this->groupClassPage, 'render' ] );
+		add_shortcode( 'us_booking', self::shortcode( [ $this->bookingPage, 'render' ] ) );
+		add_shortcode( 'us_student_login', self::shortcode( [ $this->loginPage, 'render' ] ) );
+		add_shortcode( 'us_student_register', self::shortcode( [ $this->registrationPage, 'render' ] ) );
+		add_shortcode( 'us_group_classes', self::shortcode( [ $this->groupClassPage, 'render' ] ) );
 		add_action( 'template_redirect', [ $this->registrationPage, 'maybeRedirectToRegistrationPage' ] );
 		add_action( 'wp_enqueue_scripts', [ $this, 'enqueueAssets' ] );
 	}
 
+	/**
+	 * Wraps a page renderer so bare shortcode usage is safe: WordPress passes
+	 * an empty string, not an array, to the callback when a shortcode is used
+	 * without attributes (`shortcode_parse_atts( '' )` returns `''`).
+	 *
+	 * @param callable(array<int|string, mixed>): string $render
+	 * @return \Closure(mixed): string
+	 */
+	private static function shortcode( callable $render ): \Closure {
+		return static fn( mixed $atts ): string => $render( is_array( $atts ) ? $atts : [] );
+	}
+
 	public function enqueueAssets(): void {
 		wp_register_style( 'us-scheduler', USC_PLUGIN_URL . 'assets/css/frontend.css', [], USC_VERSION );