refactor(frontend): Reactivityで型を明示するように (#12791)

* refactor(frontend): Reactivityで型を明示するように

* fix: プロパティの参照が誤っているのを修正

* fix: 初期化の値を空配列に書き換えていた部分をnullに置き換え

packages/frontend/src/pages/settings/theme.manage.vue

@@ -46,7 +46,7 @@ import { definePageMetadata } from '@/scripts/page-metadata.js';
 
 const installedThemes = ref(getThemes());
 const builtinThemes = getBuiltinThemesRef();
-const selectedThemeId = ref(null);
+const selectedThemeId = ref<string | null>(null);
 
 const themes = computed(() => [...installedThemes.value, ...builtinThemes.value]);