feat(109): checkbox multiselect filters

Phase 109 complete:
- Add checkbox dropdown enhancement for statistics multi-select filters
- Preserve GET contract for channels[] and status_groups[]
- Update PAUL plan context to read .paul/codebase docs

Co-Authored-By: Codex <noreply@openai.com>

resources/views/layouts/app.php

@@ -187,6 +187,7 @@
   </div>
   <script src="/assets/js/modules/jquery-alerts.js?ver=<?= filemtime(dirname(__DIR__, 3) . '/public/assets/js/modules/jquery-alerts.js') ?: 0 ?>"></script>
   <script src="/assets/js/modules/global-search.js?ver=<?= filemtime(dirname(__DIR__, 3) . '/public/assets/js/modules/global-search.js') ?: 0 ?>"></script>
+  <script src="/assets/js/modules/checkbox-multiselect.js?ver=<?= filemtime(dirname(__DIR__, 3) . '/public/assets/js/modules/checkbox-multiselect.js') ?: 0 ?>"></script>
   <script>
   (function () {
     var STORAGE_KEY = 'sidebarCollapsed';

resources/views/statistics/orders.php

@@ -47,7 +47,15 @@ foreach ($channelOptions as $channelOption) {
 
     <label class="form-field">
       <span class="field-label"><?= $e($t('statistics.orders.filters.channels')) ?></span>
-      <select class="form-control statistics-orders-multiselect" name="channels[]" multiple size="6">
+      <select class="form-control statistics-orders-multiselect js-checkbox-multiselect"
+        name="channels[]"
+        multiple
+        size="6"
+        data-checkbox-multiselect
+        data-all-label="Wszystkie"
+        data-empty-label="Nic nie wybrano"
+        data-selected-label-singular="zaznaczono"
+        data-selected-label-plural="zaznaczono">
         <?php foreach ($channelOptions as $channelOption): ?>
           <?php
           $key = (string) ($channelOption['key'] ?? '');
@@ -65,7 +73,15 @@ foreach ($channelOptions as $channelOption) {
 
     <label class="form-field">
       <span class="field-label"><?= $e($t('statistics.orders.filters.status_groups')) ?></span>
-      <select class="form-control statistics-orders-multiselect" name="status_groups[]" multiple size="6">
+      <select class="form-control statistics-orders-multiselect js-checkbox-multiselect"
+        name="status_groups[]"
+        multiple
+        size="6"
+        data-checkbox-multiselect
+        data-all-label="Wszystkie"
+        data-empty-label="Nic nie wybrano"
+        data-selected-label-singular="zaznaczono"
+        data-selected-label-plural="zaznaczono">
         <?php foreach ($statusGroupOptions as $groupOption): ?>
           <?php
           $groupId = (int) ($groupOption['id'] ?? 0);