实例顶栏增加退出登录按钮，便于清会话回到登录页。

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/common/static/instance_theme.css

@@ -626,6 +626,39 @@ html[data-theme="light"] .theme-toggle-btn.is-active {
   margin-left: auto;
 }
 
+.instance-logout-btn {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 5px 12px;
+  font-size: 0.82rem;
+  line-height: 1.2;
+  color: var(--inst-muted, #9aa3b2);
+  background: transparent;
+  border: 1px solid rgba(255, 255, 255, 0.14);
+  border-radius: 8px;
+  text-decoration: none;
+  white-space: nowrap;
+  cursor: pointer;
+}
+
+.instance-logout-btn:hover {
+  color: var(--inst-text, #e8ecf4);
+  border-color: rgba(255, 255, 255, 0.28);
+  background: rgba(255, 255, 255, 0.04);
+}
+
+html[data-theme="light"] .instance-logout-btn {
+  color: #5b6472;
+  border-color: rgba(0, 0, 0, 0.12);
+}
+
+html[data-theme="light"] .instance-logout-btn:hover {
+  color: #1f2937;
+  border-color: rgba(0, 0, 0, 0.22);
+  background: rgba(0, 0, 0, 0.03);
+}
+
 .instance-header-toolbar-filter label {
   display: inline-flex;
   align-items: center;