fix: activated theme not loaded after initialization (#790)

#### What type of PR is this?

/kind bug

#### What this PR does / why we need it:

修复首次初始化之后，Console 没有加载已激活主题的问题。

#### Which issue(s) this PR fixes:

Fixes halo-dev/halo#3018

#### Special notes for your reviewer:

测试方式：

1. 使用全新的环境
2. 参考 https://docs.halo.run/developer-guide/core/build 构建可执行 JAR。
3. 运行 Halo，初始化之后检查 Console 主题页面是否已经加载了已经激活的主题。

#### Does this PR introduce a user-facing change?

```release-note
修复首次初始化之后，Console 没有加载已激活主题的问题。
```

src/views/system/Setup.vue

@@ -18,6 +18,7 @@ import type {
   SinglePageRequest,
   Tag,
 } from "@halo-dev/api-client";
+import { useThemeStore } from "@/stores/theme";
 
 const router = useRouter();
 
@@ -98,6 +99,8 @@ const handleSubmit = async () => {
 
     const systemStateStore = useSystemStatesStore();
     await systemStateStore.fetchSystemStates();
+    const themeStore = useThemeStore();
+    await themeStore.fetchActivatedTheme();
 
     router.push({ name: "Dashboard" });