fix: make titlebar workspace toggle clickable and add floating expand button

src/renderer/css/app.css

@@ -45,9 +45,9 @@ body {
 .titlebar-left {
   display: flex;
   align-items: center;
-  gap: 8px;
+  gap: 6px;
   overflow: hidden;
-  -webkit-app-region: drag; /* 允许左侧空白和品牌区域拖拽 */
+  -webkit-app-region: no-drag; /* 允许子元素正常响应鼠标交互 */
 }
 
 .app-brand {
@@ -61,6 +61,7 @@ body {
   letter-spacing: 0.5px;
   -webkit-app-region: drag;
   cursor: grab;
+  flex-shrink: 0;
 }
 
 .titlebar-drag-spacer {
@@ -95,6 +96,8 @@ body {
   justify-content: center;
   cursor: pointer;
   transition: all 0.15s ease;
+  -webkit-app-region: no-drag !important;
+  flex-shrink: 0;
 }
 
 .btn-icon:hover {