Extract post list into reusable fragment

Add public/fragments/post-list.html containing the shared post list markup, and update src/pages/category.astro and src/pages/index.astro to replace the in-file duplicate list with <th:block th:replace="~{fragments/post-list}"></th:block>. This removes duplicated template code and centralizes the post list for easier maintenance; existing empty-feed and pagination logic remain unchanged.

public/fragments/post-list.html

@@ -0,0 +1,19 @@
+<ul class="post-feed__list" role="list">
+  <li th:each="post : ${posts.items}" class="post-feed__item">
+    <a class="post-feed__link" th:href="@{${post.status.permalink}}">
+      <div class="post-feed__body">
+        <p
+          class="post-feed__date"
+          th:text="${#dates.format(post.spec.publishTime, 'yyyy-MM-dd')}"
+        ></p>
+        <h2 class="post-feed__title" th:text="${post.spec.title}"></h2>
+        <p class="post-feed__excerpt" th:text="${post.status.excerpt}"></p>
+      </div>
+      <img
+        th:unless="${#strings.isEmpty(post.spec.cover)}"
+        class="post-feed__image"
+        th:src="${post.spec.cover}"
+      />
+    </a>
+  </li>
+</ul>