fix(genre-filter): hide carets for genres without children

Only show expand carets in the advanced view for genres that actually
have children loaded, or where children haven't been loaded yet.
Applies to all levels (parent, child, grandchild).

src/lib/components/GenreFilterPopup.svelte

@@ -295,15 +295,19 @@
 
             <div class="tree-node">
               <div class="node-row level-0" class:selected={parentState === 'all'} class:partial={parentState === 'partial'}>
-                <button class="expand-btn" onclick={(e) => { e.stopPropagation(); toggleExpanded(parentNode.genre.id); }} type="button" title={isExpanded ? 'Collapse' : 'Expand'}>
-                  {#if isLoading}
-                    <Loader2 size={14} class="animate-spin" />
-                  {:else if isExpanded}
-                    <ChevronDown size={14} />
-                  {:else}
-                    <ChevronRight size={14} />
-                  {/if}
-                </button>
+                {#if hasLoadedChildren || !areChildrenLoaded(parentNode.genre.id)}
+                  <button class="expand-btn" onclick={(e) => { e.stopPropagation(); toggleExpanded(parentNode.genre.id); }} type="button" title={isExpanded ? 'Collapse' : 'Expand'}>
+                    {#if isLoading}
+                      <Loader2 size={14} class="animate-spin" />
+                    {:else if isExpanded}
+                      <ChevronDown size={14} />
+                    {:else}
+                      <ChevronRight size={14} />
+                    {/if}
+                  </button>
+                {:else}
+                  <span class="expand-placeholder"></span>
+                {/if}
 
                 <button class="node-content" onclick={() => handleNodeToggle(parentNode.genre.id)} type="button">
                   <span class="check-box" class:checked={parentState === 'all'} class:partial={parentState === 'partial'}>
@@ -331,7 +335,7 @@
 
                     <div class="tree-node">
                       <div class="node-row level-1" class:selected={childState === 'all'} class:partial={childState === 'partial'}>
-                        {#if hasGrandchildren || !areChildrenLoaded(childNode.genre.id)}
+                        {#if hasGrandchildren}
                           <button class="expand-btn" onclick={(e) => { e.stopPropagation(); toggleExpanded(childNode.genre.id); }} type="button" title={childExpanded ? 'Collapse' : 'Expand'}>
                             {#if childLoading}
                               <Loader2 size={12} class="animate-spin" />