feat(theme): 提供i18n能力

theme-terminal/templates/modules/category-tree.html

@@ -1,7 +1,7 @@
 <ul th:fragment="next (categories)">
   <li th:fragment="single (categories)" th:each="category : ${categories}">
     <a th:href="@{${category.status.permalink}}">
-      <span th:text="${category.spec.displayName}+' ('+${category.status.visiblePostCount}+' 篇文章)'"> </span>
+      <span th:text="${category.spec.displayName}+' ('+${category.status.visiblePostCount?:0}+' 篇文章)'"> </span>
     </a>
     <th:block th:if="${not #lists.isEmpty(category.children)}">
       <th:block th:replace="~{modules/category-tree :: next (categories=${category.children})}"></th:block>