fix(frontend): 切换目录时只保留一次虚拟环境状态

切换目录时先同步清掉旧目录自动填入的 venv；实时 venv 为空时立即忽略防抖旧值；旧查询结果不再回填新目录；补充 Playwright 回归用例。

frontend/e2e/dashboard.spec.ts

@@ -204,6 +204,33 @@ test('switches script language and adapts the form fields', async ({ page }) =>
   await dialog.getByRole('button', { name: '取消' }).click()
 })
 
+test('avoids a transient invalid venv when switching directories', async ({
+  page,
+}) => {
+  const venvDir = createVenvFixture()
+  const emptyProject = join(venvDir, '..', 'empty-project')
+  mkdirSync(emptyProject, { recursive: true })
+
+  await page.goto('/')
+  await page.getByRole('button', { name: '+ 新建任务' }).click()
+  const dialog = page.getByRole('dialog', { name: '新建定时任务' })
+  const workingDir = dialog.getByLabel(/项目工作目录/)
+  const bar = dialog.locator('.env-detector-bar')
+
+  await workingDir.fill(venvDir)
+  await expect(bar).toContainText('已关联虚拟环境')
+
+  await workingDir.fill(emptyProject)
+  const states = new Set<string>()
+  for (let index = 0; index < 30; index += 1) {
+    states.add(await bar.innerText())
+    await page.waitForTimeout(80)
+  }
+
+  expect([...states].some((text) => text.includes('缺少'))).toBe(false)
+  await expect(bar).toContainText('未检测到虚拟环境')
+})
+
 test('keeps the dashboard usable on a narrow viewport', async ({
   page,
   request,