feat(update-center): 添加加载状态处理及UI优化

为更新中心添加加载状态管理，包括：
- 在打开和刷新操作时显示加载状态
- 禁用刷新按钮防止重复操作
- 添加加载中的动画效果和提示文本
- 优化加载时的UI显示

src/components/UpdateCenterModal.vue

@@ -22,6 +22,7 @@
           :selected-count="selectedCount"
           :all-selected="store.allSelected.value"
           :some-selected="store.someSelected.value"
+          :loading="store.loading.value"
           @refresh="store.refresh"
           @start-selected="emit('request-start-selected')"
           @request-close="store.requestClose"
@@ -42,16 +43,33 @@
           </p>
         </div>
 
-        <div class="flex min-h-0 flex-1">
-          <UpdateCenterList
-            :items="store.filteredItems.value"
-            :tasks="store.snapshot.value.tasks"
-            :selected-task-keys="store.selectedTaskKeys.value"
-            @toggle-selection="emit('toggle-selection', $event)"
-            @ignore-item="store.ignoreItem"
-            @unignore-item="store.unignoreItem"
-          />
+        <div
+          v-if="store.loading.value && store.filteredItems.value.length === 0"
+          class="flex min-h-0 flex-1 items-center justify-center p-6"
+        >
+          <div class="flex flex-col items-center gap-3 text-slate-500 dark:text-slate-400">
+            <i class="fas fa-circle-notch fa-spin text-3xl"></i>
+            <p class="text-sm">正在检查更新…</p>
+          </div>
         </div>
+        <template v-else>
+          <div
+            v-if="store.loading.value && store.filteredItems.value.length > 0"
+            class="border-b border-slate-200/70 px-6 py-2 text-center text-xs text-slate-400 dark:border-slate-800/70 dark:text-slate-500"
+          >
+            正在刷新更新列表…
+          </div>
+          <div class="flex min-h-0 flex-1">
+            <UpdateCenterList
+              :items="store.filteredItems.value"
+              :tasks="store.snapshot.value.tasks"
+              :selected-task-keys="store.selectedTaskKeys.value"
+              @toggle-selection="emit('toggle-selection', $event)"
+              @ignore-item="store.ignoreItem"
+              @unignore-item="store.unignoreItem"
+            />
+          </div>
+        </template>
 
         <UpdateCenterMigrationConfirm
           :show="store.showMigrationConfirm.value"