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/__tests__/unit/update-center/UpdateCenterModal.test.ts

@@ -63,6 +63,7 @@ const createStore = (
 
   return {
     isOpen: ref(true),
+    loading: ref(false),
     showCloseConfirm: ref(true),
     showMigrationConfirm: ref(false),
     searchQuery: ref(""),
@@ -220,4 +221,52 @@ describe("UpdateCenterModal", () => {
 
     expect(store.requestClose).toHaveBeenCalledTimes(1);
   });
+
+  it("shows loading panel when loading with no items", () => {
+    const store = createStore({
+      items: [],
+      tasks: [],
+      warnings: [],
+      hasRunningTasks: false,
+    });
+    store.loading.value = true;
+
+    render(UpdateCenterModal, {
+      props: {
+        show: true,
+        store,
+      },
+    });
+
+    expect(screen.getByText("正在检查更新…")).toBeTruthy();
+  });
+
+  it("shows refresh hint while loading with existing items", () => {
+    const store = createStore({ hasRunningTasks: false });
+    store.loading.value = true;
+
+    render(UpdateCenterModal, {
+      props: {
+        show: true,
+        store,
+      },
+    });
+
+    expect(screen.getByText("Spark Weather")).toBeTruthy();
+    expect(screen.getByText("正在刷新更新列表…")).toBeTruthy();
+  });
+
+  it("disables refresh button while loading", () => {
+    const store = createStore({ hasRunningTasks: false });
+    store.loading.value = true;
+
+    render(UpdateCenterModal, {
+      props: {
+        show: true,
+        store,
+      },
+    });
+
+    expect(screen.getByRole("button", { name: /刷新/ })).toBeDisabled();
+  });
 });