fix: restore rounded workbar tabs

The workbar tabs, close controls, and add button now use the shared rounded control styling again. The terminal panel and its toolbar remain square as requested.

Release note: 恢复工作栏页签、关闭按钮和加号入口的圆角外观；终端面板与终端工具栏继续保持直角。

src/renderer/src/WorkbarShell.test.tsx

@@ -72,12 +72,12 @@ function ControlledShell({
 }
 
 describe('WorkbarShell', () => {
-  it('keeps tabs and their controls square', () => {
+  it('keeps tabs and their controls rounded', () => {
     expect(stylesheet).toMatch(
-      /\.workbar-shell__tab-item\s*\{[^}]*border-radius:\s*0;/u
+      /\.workbar-shell__tab-item\s*\{[^}]*border-radius:\s*var\(--radius-control\) var\(--radius-control\) 0 0;/u
     )
     expect(stylesheet).toMatch(
-      /\.workbar-shell__tab-close,\s*\n\.workbar-shell__add\s*\{[^}]*border-radius:\s*0;/u
+      /\.workbar-shell__tab-close,\s*\n\.workbar-shell__add\s*\{[^}]*border-radius:\s*var\(--radius-control\);/u
     )
   })
 

src/renderer/src/styles.css

@@ -3858,7 +3858,7 @@ button > svg {
   align-items: center;
   border: 1px solid transparent;
   border-bottom: 0;
-  border-radius: 0;
+  border-radius: var(--radius-control) var(--radius-control) 0 0;
   background: transparent;
   transition:
     border-color var(--motion-fast) ease-out,
@@ -3936,7 +3936,7 @@ button > svg {
   height: 28px;
   padding: 0;
   place-items: center;
-  border-radius: 0;
+  border-radius: var(--radius-control);
   background: transparent;
   color: var(--text-muted);
   cursor: pointer;