test(desktop): fix day-divider strict-mode failure across midnight UTC

The general-channel welcome seeds are backdated by up to 120s, so a smoke
run that straddles midnight UTC renders two day dividers (Yesterday +
Today). Three specs asserted toBeVisible() on the bare
message-timeline-day-divider locator, which Playwright strict mode
rejects the moment two dividers exist — this is what failed Desktop
Smoke E2E shard 3 on the 23:53 UTC run of PR #6003 (test started before
midnight, assertion ran after).

Assert .first() visibility instead at all three sites (messaging.spec.ts
day-divider test, channels.spec.ts general-channel content test, and the
DM unread-clear test). The tests' intent is "a divider appears", which
the first divider proves on both sides of midnight. Pre-existing on
main; surfaced here because the PR run happened to cross the boundary.

Co-authored-by: Thomas Petersen <thomasp@squareup.com>
Signed-off-by: Thomas Petersen <thomasp@squareup.com>

desktop/tests/e2e/channels.spec.ts

@@ -1869,7 +1869,11 @@ test("channel with messages shows content", async ({ page }) => {
   await expect(page.getByTestId("welcome-composer-guide-banner")).toHaveCount(
     0,
   );
-  await expect(page.getByTestId("message-timeline-day-divider")).toBeVisible();
+  // `.first()`: backdated seeds can straddle midnight UTC and render two
+  // dividers (Yesterday + Today); a bare locator fails Playwright strict mode.
+  await expect(
+    page.getByTestId("message-timeline-day-divider").first(),
+  ).toBeVisible();
   await expect(page.getByTestId("message-timeline")).toContainText(
     "Welcome to general",
   );
@@ -2441,7 +2445,11 @@ test("sidebar clears unread indicator after opening a DM", async ({ page }) => {
   await expect(page.getByTestId("message-dm-intro")).toContainText(
     "This is the beginning of your direct message with",
   );
-  await expect(page.getByTestId("message-timeline-day-divider")).toBeVisible();
+  // `.first()`: backdated seeds can straddle midnight UTC and render two
+  // dividers (Yesterday + Today); a bare locator fails Playwright strict mode.
+  await expect(
+    page.getByTestId("message-timeline-day-divider").first(),
+  ).toBeVisible();
   await expect(page.getByTestId("message-timeline")).toContainText(
     "Unread update for the DM",
   );

desktop/tests/e2e/messaging.spec.ts

@@ -1996,7 +1996,13 @@ test("day divider appears in timeline", async ({ page }) => {
   await expect(page.getByTestId("message-timeline")).toContainText(
     "Welcome to general",
   );
-  await expect(page.getByTestId("message-timeline-day-divider")).toBeVisible();
+  // `.first()`: the seeds are backdated by up to 120s, so a run that straddles
+  // midnight UTC legitimately renders two dividers (Yesterday + Today) and a
+  // bare locator fails Playwright strict mode. The intent is "a divider
+  // appears", which the first divider proves on both sides of midnight.
+  await expect(
+    page.getByTestId("message-timeline-day-divider").first(),
+  ).toBeVisible();
 });
 
 test("send message to DM channel p-tags the recipient", async ({ page }) => {