fix(e2e): harden card selectors for session resume

Co-Authored-By: Codex <noreply@openai.com>

frontend/e2e/library-start.spec.ts

@@ -24,7 +24,7 @@ test.describe('flow library start-session smoke tests', () => {
       await page.getByPlaceholder('Search flows...').fill(tree.name)
       await page.getByRole('button', { name: 'Search', exact: true }).click()
 
-      const treeCard = page.locator('.bg-card').filter({ hasText: tree.name }).first()
+      const treeCard = page.getByTestId('tree-card').filter({ hasText: tree.name }).first()
       await expect(treeCard).toBeVisible()
       await treeCard.getByRole('button', { name: /^Start(?: Session)?$/ }).click()