e2e: close the interact-before-hydration race (cold-start flake)

Diagnosis (reproduced by looping cold-server runs): the recurring
"cold start" failures in the auth/follow specs were a hydration race,
not a Vite dep-reload (the server.warmup config already handles that —
no reload events in the logs). FollowButton is an onClick button, and
Playwright considers it actionable (visible + enabled) before React
attaches the handler on a freshly-navigated page. A click in that
window is dropped (or triggers a native form submit), so the expected
state change never happens — e.g. notifications.test.ts:59 clicked
"Request to follow" with no hydration wait and the button never
flipped to "Requested".

The existing waitForHydration guard was applied per-interaction and so
was easy to forget (test 1 had it, test 3 didn't). Fix it at the
navigation instead:

- add gotoHydrated(page, url) = goto + waitForHydration, documented as
  the default for "navigate then interact with a React control"
- use it before every FollowButton interaction in notifications +
  social
- consolidate the setProfileVisibility helper (copy-pasted in three
  specs, two missing the hydration wait) into e2e/helpers/profile.ts,
  built on gotoHydrated

Validation against a fresh Postgres, cold servers: notifications 0/10
failures (was the proven flaker); notifications+social+explore 0/5
(55 tests); planner unaffected (25/25).

Note: two other "cold-start"-attributed flakes are NOT this race and
are out of scope here — planner map-load latency (mitigated by warmup
+ CI retries) and the /explore directory assertion (a local artifact
of the scratch DB accumulating >20 public users; CI's fresh DB stays
under the page size, so it doesn't bite there).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

e2e/explore.test.ts

@@ -1,15 +1,6 @@
-import { test, expect, waitForHydration, type CDPSession, type Page } from "./fixtures/test";
+import { test, expect, type CDPSession } from "./fixtures/test";
 import { setupVirtualAuthenticator, registerUser } from "./helpers/auth";
-
-async function setProfileVisibility(page: Page, value: "public" | "private") {
-  await page.goto("/settings/profile");
-  // Wait for hydration so the visibility form submits via the fetcher
-  // (and shows the "Profile saved." toast) instead of native-navigating.
-  await waitForHydration(page);
-  await page.locator(`input[type=radio][name=profileVisibility][value=${value}]`).check();
-  await page.getByRole("button", { name: /^Save$/ }).first().click();
-  await expect(page.getByText("Profile saved.")).toBeVisible({ timeout: 10000 });
-}
+import { setProfileVisibility } from "./helpers/profile";
 
 test.describe.configure({ mode: "serial" });