Auto-redirect invite tokens to the registration page; rename invite button

- RegistrationPage::maybeRedirectToRegistrationPage() (hooked on
  template_redirect): any front-end request carrying a us_invite token is
  redirected to the configured registration page (token preserved), unless
  already there. Covers links shared before a page was selected; no-op when
  no page is set.
- Invites button text: "Send Invite" -> "Generate Invitation Link".
- Doc updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ShortcodeRegistrar.php

@@ -32,6 +32,7 @@ class ShortcodeRegistrar {
 		add_shortcode( 'us_booking', [ $this->bookingPage, 'render' ] );
 		add_shortcode( 'us_student_login', [ $this->loginPage, 'render' ] );
 		add_shortcode( 'us_student_register', [ $this->registrationPage, 'render' ] );
+		add_action( 'template_redirect', [ $this->registrationPage, 'maybeRedirectToRegistrationPage' ] );
 		add_action( 'wp_enqueue_scripts', [ $this, 'enqueueAssets' ] );
 	}