Enable concurrent vitest execution (#36998)

Enable
[`sequence.concurrent`](https://vitest.dev/config/sequence.html#sequence-concurrent)
to run all js tests in parallel. This will help catch potential
concurrency bugs in the future. The "Repository Branch Settings" test
was not concurrency-safe, it was refactored to remove shared mutable
state.

Co-Authored-By: Claude (claude-opus-4-6) <noreply@anthropic.com>

---------

Co-authored-by: Claude (claude-opus-4-6) <noreply@anthropic.com>

web_src/js/markup/render-iframe.test.ts

@@ -28,6 +28,7 @@ describe('navigateToIframeLink', () => {
   });
 
   test('unsafe links', () => {
+    const errorSpy = vi.spyOn(console, 'error').mockImplementation(() => undefined);
     window.location.href = 'http://localhost:3000/';
 
     // eslint-disable-next-line no-script-url
@@ -41,6 +42,7 @@ describe('navigateToIframeLink', () => {
     expect(openSpy).toHaveBeenCalledTimes(0);
     expect(assignSpy).toHaveBeenCalledTimes(0);
     expect(window.location.href).toBe('http://localhost:3000/');
+    errorSpy.mockRestore();
     vi.clearAllMocks();
   });
 });