Fix PHPStan level 6 errors from CI

- Remove unused AvailabilityRepository/BookingRepository from BookingPage
  and ShortcodeRegistrar (template is a JS shell; no PHP data needed yet)
- Add @param array<string, string> to shortcode render() signatures
- Add @return array<string, mixed> to model toArray() methods
- Fix get_permalink() ?? '' — returns string|false not nullable, use cast
- Remove unused ignoreErrors pattern from phpstan.neon

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Frontend/LoginPage.php

@@ -5,10 +5,13 @@ namespace Unsupervised\Schedular\Frontend;
 
 class LoginPage
 {
+    /**
+     * @param array<string, string> $atts
+     */
     public function render(array $atts): string
     {
         if (is_user_logged_in()) {
-            $redirect = esc_url(get_permalink());
+            $redirect = esc_url((string) get_permalink());
             return sprintf(
                 '<p>%s <a href="%s">%s</a>.</p>',
                 esc_html__('You are already logged in.', 'unsupervised-schedular'),
@@ -18,7 +21,7 @@ class LoginPage
         }
 
         $error    = '';
-        $redirect = sanitize_url(get_permalink() ?? '');
+        $redirect = sanitize_url((string) get_permalink());
 
         if (isset($_POST['us_login']) && check_admin_referer('us_student_login')) {
             $credentials = [