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>

templates/admin/invites.php

@@ -55,7 +55,7 @@ if (! defined('ABSPATH')) {
                 <td><input type="email" name="email" id="email" class="regular-text" required></td>
             </tr>
         </table>
-        <?php submit_button(esc_html__('Send Invite', 'unsupervised-schedular')); ?>
+        <?php submit_button(esc_html__('Generate Invitation Link', 'unsupervised-schedular')); ?>
     </form>
 
     <h2><?php esc_html_e('Pending Invites', 'unsupervised-schedular'); ?></h2>