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/static/css/common.css

@@ -145,6 +145,27 @@ header {
     color: var(--color-secondary) !important;
 }
 
+/* ---------- Nav Contribute ---------- */
+.nav-contribute {
+    display: inline-flex !important;
+    align-items: center;
+    vertical-align: middle;
+    line-height: 1;
+    color: var(--color-accent) !important;
+    font-weight: 600 !important;
+}
+
+.nav-contribute .icon {
+    width: 16px;
+    height: 16px;
+    display: block;
+    margin-right: 4px;
+}
+
+.nav-contribute:hover {
+    opacity: .8;
+}
+
 /* ---------- Nav Messages Bell ---------- */
 .nav-messages {
     position: relative;