feat: NAV 添加投稿入口，笔图标+投稿

- nav.html: 帖子后添加笔图标+投稿链接（仅登录可见），跳转 /studio/write
- posts/index.html: 移除独立的撰写帖子按钮
- common.css: 添加 .nav-contribute 样式，inline-flex + vertical-align: middle + line-height: 1

templates/MetaLab-2026/html/layout/nav.html

@@ -8,6 +8,10 @@
             <li><a href="/">首页</a></li>
             <li><a href="/posts">帖子</a></li>
             {{if .IsLoggedIn}}
+            <li><a href="/studio/write" class="nav-contribute">
+                <svg class="icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M12 20h9"/><path d="M16.5 3.5a2.121 2.121 0 0 1 3 3L7 19l-4 1 1-4L16.5 3.5z"/></svg>
+                投稿
+            </a></li>
             <li>
                 <a href="/messages" class="nav-messages" id="messagesBell" title="消息中心">
                     <svg class="nav-bell-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M18 8A6 6 0 0 0 6 8c0 7-3 9-3 9h18s-3-2-3-9"/><path d="M13.73 21a2 2 0 0 1-3.46 0"/></svg>