🐛 fix(tabs/theme): 修复标签顶条与标题栏主题不一致

- 移除 V2 工作区活动标签顶部强调条，保留圆角与选中背景
- 让 V2 自定义标题栏跟随侧栏主题表面，同时保持 legacy 背景逻辑
- 补充标签与标题栏主题回归断言

frontend/src/components/TabManager.adaptive-width.test.ts

@@ -52,12 +52,12 @@ describe('v2 workbench adaptive tab width', () => {
     );
   });
 
-  it('rounds all four corners of every v2 workbench tab', () => {
+  it('rounds all four corners without drawing an accent bar above the active tab', () => {
     expect(themeSource).toMatch(
       /\.gn-v2-main-tabs \.ant-tabs-tab \{[^}]*border-radius: 8px !important;/s,
     );
-    expect(themeSource).toMatch(
-      /\.gn-v2-main-tabs \.ant-tabs-tab\.ant-tabs-tab-active \{[^}]*box-shadow: inset 0 2px 0 var\(--gn-accent\) !important;/s,
+    expect(themeSource).not.toMatch(
+      /\.gn-v2-main-tabs \.ant-tabs-tab\.ant-tabs-tab-active \{[^}]*box-shadow:/s,
     );
     expect(themeSource).toMatch(
       /\.gn-v2-main-tabs \.ant-tabs-tab\.ant-tabs-tab-active::after \{[^}]*display: none;/s,