fix: 补充图库未选中布局背景

为未选中的网格或瀑布流按钮增加浅灰底色，保留选中绿色状态，并补充切换状态测试。

src/features/subject-images/SubjectImagesPage.vue

@@ -584,7 +584,8 @@ async function showStaleForms() {
                                 <span class="filter-result-count">{{ filtered.length }} 个形态</span>
                                 <div class="gallery-layout-switch" role="group" aria-label="图库显示方式">
                                     <NButton
-                                        class="icon-button"
+                                        class="icon-button layout-option-button"
+                                        :class="{ 'layout-option-active': layout === 'grid' }"
                                         size="small"
                                         :type="layout === 'grid' ? 'primary' : 'default'"
                                         :aria-pressed="layout === 'grid'"
@@ -595,7 +596,8 @@ async function showStaleForms() {
                                         <template #icon><LayoutGrid :size="16" /></template>
                                     </NButton>
                                     <NButton
-                                        class="icon-button"
+                                        class="icon-button layout-option-button"
+                                        :class="{ 'layout-option-active': layout === 'masonry' }"
                                         size="small"
                                         :type="layout === 'masonry' ? 'primary' : 'default'"
                                         :aria-pressed="layout === 'masonry'"
@@ -898,6 +900,10 @@ async function showStaleForms() {
 .gallery-layout-switch {
     @apply flex items-center gap-1;
 }
+/* 未选中的排版按钮保留浅灰底，和筛选容器区分，同时不覆盖选中项的绿色状态。 */
+.gallery-layout-switch .layout-option-button:not(.layout-option-active) {
+    background: var(--app-control-hover);
+}
 @container (max-width: 1200px) {
     .form-image-toolbar.has-impact-picker,
     .form-image-toolbar.has-impact-picker.has-impact-actions {