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/Auth/RegistrationPage.php

@@ -52,6 +52,31 @@ class RegistrationPage {
 		return (string) ob_get_clean();
 	}
 
+	/**
+	 * Redirect to the configured registration page when an invite token lands
+	 * elsewhere (e.g. a link generated before the page was selected). Hooked on
+	 * `template_redirect`.
+	 */
+	public function maybeRedirectToRegistrationPage(): void {
+		if ( is_admin() ) {
+			return;
+		}
+
+		// phpcs:ignore WordPress.Security.NonceVerification.Recommended -- read-only token used only to build the redirect target.
+		$token = sanitize_text_field( wp_unslash( $_GET['us_invite'] ?? '' ) );
+		if ( '' === $token ) {
+			return;
+		}
+
+		$pageId = (int) get_option( RegistrationController::OPTION_PAGE, 0 );
+		if ( $pageId <= 0 || is_page( $pageId ) ) {
+			return;
+		}
+
+		wp_safe_redirect( add_query_arg( 'us_invite', rawurlencode( $token ), (string) get_permalink( $pageId ) ) );
+		exit;
+	}
+
 	/**
 	 * Process the submitted registration. Returns true on success or an error
 	 * message string on failure.