fix: restore full workbar resize hit area

Keep the workbar divider's 9px hit target inside the sidebar so overflow clipping no longer makes it difficult to drag. Reserve matching space beside browser and terminal content, and remove it in browser fullscreen mode.

Validation: 68 sidebar tests, typecheck, lint, and Chromium hit tests passed. Full suite timed out after 10 minutes and reported three supervision test failures.

Release note: Make the divider between chat and the browser or terminal workbar easier to grab by restoring its full resize area.

UI-DESIGN.md

@@ -380,6 +380,8 @@
   遮罩或隔离其内容。分栏计算排除最左侧主导航，主工作区与工作栏采用相同的 `300px`
   最小宽度；拖动保存两者在可用分栏宽度中的比例，任一侧到达最小值后都不得继续侵占。
   所有可容纳两侧最小值的尺寸均支持指针和键盘调整，并须保留全部栏目入口。
+- 工作栏分隔条须保留完整的 `9px` 拖拽命中区域，位于侧栏内部，不能被容器裁剪；栏目内容
+  和原生浏览器视口须避开该区域。浏览器全屏时隐藏分隔条并取消这段预留空间。
 - 工作栏开关固定在顶栏 `.topbar__actions` 内、紧邻浅色/深色主题按钮之前，同一个按钮负责展开与收起并持续暴露状态；图标中的
   箭头方向必须随当前动作切换，收起状态提示展开，展开状态提示收起。按钮保持普通图标按钮
   视觉，不通过持续高亮表达展开状态。工作栏内部不再重复关闭按钮；当前会话没有 Task 时，

src/renderer/src/styles.css

@@ -4021,9 +4021,9 @@ button > svg {
   z-index: 2;
   top: 0;
   bottom: 0;
-  left: -8px;
+  left: 0;
   display: none;
-  width: 12px;
+  width: 9px;
   padding: 0;
   cursor: col-resize;
   touch-action: none;
@@ -4041,7 +4041,7 @@ button > svg {
   position: absolute;
   top: 0;
   bottom: 0;
-  left: 7px;
+  left: 0;
   width: 2px;
   background: transparent;
   content: '';
@@ -4059,6 +4059,15 @@ button > svg {
   outline-offset: -2px;
 }
 
+.assistant-sidebar--open > .workbar-shell {
+  /* Keep native browser views and terminal content outside the resize target. */
+  margin-left: 9px;
+}
+
+.assistant-sidebar--browser-fullscreen > .workbar-shell {
+  margin-left: 0;
+}
+
 .workbar-shell {
   display: flex;
   min-width: 0;