fix: address code quality issues in category assignment modal

- Add btn--sm class to assign-categories button
- Force DOM reflow in openModal() so CSS transition fires correctly
- Clear treeEl.innerHTML at start of renderTree() to prevent stale tree
  on rapid double-click (race between two parallel fetch pairs)

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

resources/views/marketplace/offers.php

@@ -53,7 +53,7 @@
               <td>
                 <button
                   type="button"
-                  class="btn btn--secondary js-assign-categories"
+                  class="btn btn--secondary btn--sm js-assign-categories"
                   data-integration-id="<?= $e((string) $integrationId) ?>"
                   data-product-id="<?= $e((string) ($row['external_product_id'] ?? '')) ?>"
                 >Przypisz kategorie</button>
@@ -111,6 +111,7 @@
 
   function openModal() {
     backdrop.style.display = '';
+    backdrop.getBoundingClientRect(); // force reflow so CSS transition fires
     backdrop.setAttribute('aria-hidden', 'false');
     backdrop.classList.add('is-visible');
     loadingEl.style.display = '';
@@ -224,6 +225,7 @@
   }
 
   function renderTree(flat, checked) {
+    treeEl.innerHTML = '';
     var roots = buildTree(flat);
     var ul = document.createElement('ul');
     ul.style.cssText = 'padding:0;margin:0';