test(desktop): wait for channel head refresh before paging thread summary test (#7955)

🤖

## Summary
Main's Desktop Smoke E2E (4) job fails on `scroll-history.spec.ts`
("thread summary badge survives a retained older-history prepend"). This
change fixes the test's timing. It does not change app code.

When a channel opens, the app starts a live subscription. When that
subscription is ready, the app fetches the newest page again, and that
fetch replaces the whole loaded history. This test scrolled up before
that refresh happened. On slower CI runners, the refresh came after the
older page loaded but before it showed on screen. The older rows went
away, the reader stayed at the top, and no more pages loaded. All three
attempts failed that way.

The test now calls `waitForMockChannelHeadReady` before it scrolls, like
the other pagination tests in this file already do. That helper waits
for the live subscription and its refresh to finish.

### Related issue
Main CI failure at `15d44dcc`
(https://github.com/block/buzz/actions/runs/36470599751/job/109092076235).
The same failure was reported on #7827.

### Testing
- Added temporary logs to see the order of events. Locally, the refresh
ran after the older page had loaded: the loaded history went from 2
pages back to 1. The test passed only because it checked before the next
render. With the wait, the refresh runs first and nothing replaces the
loaded pages later.
- `playwright test --project=smoke tests/e2e/scroll-history.spec.ts`: 18
passed. This test with `--repeat-each=8`: 8 passed.

### Follow-up (not in this PR)
The app has the same race for real users. If someone scrolls up before
the live subscription is ready, or during a reconnect, the refresh can
drop the older history they loaded. If they are at the top of the list,
paging stops until they scroll down and back up.

Signed-off-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>

desktop/tests/e2e/scroll-history.spec.ts

@@ -2172,6 +2172,13 @@ test("thread summary badge survives a retained older-history prepend", async ({
 
   await page.getByTestId("channel-deep-history").click();
   await expect(page.getByTestId("chat-title")).toHaveText("deep-history");
+  // Page only after live admission and its head refresh. That refresh
+  // replaces the window, so an earlier prepend can be discarded.
+  await waitForMockChannelHeadReady(
+    page,
+    "deep-history",
+    "feedf00d-0000-4000-8000-000000000007",
+  );
   const timeline = page.getByTestId("message-timeline");
   const badgeSelector =
     '[data-testid="message-thread-summary"][data-thread-head-id="mock-deep-history-599"]';