fix(auth): a tab acts only as the account it was built for, and the review's other findings

A tab names the account it was claimed for on every request, beacon and upload
(X-Siftlode-Expect-Account); current_user and /auth/logout refuse a mismatch with
401 account_changed and keep the session, so a tab left showing the account
signed in before reloads instead of writing to the new one.

/auth/login clears a connect marker left by an abandoned connect, and a link
completes only on a session that still holds. The prefs merge reads the row
under a lock. A tab's sessionStorage records its own owner. A page the account
loses is given up, address bar and remembered page included. The panel layouts
are preferences; the storage contract also refuses a key handed to the Storage
API or to usePref outside its kind. A refused sign-in as someone else and a
failed sign-out say so. The chromium-only e2e specs are in the gate register,
the member walk waits for its requests, and devices.spec checks the save itself.

frontend/e2e/devices.spec.ts

@@ -39,6 +39,12 @@ test("a preference follows the member to another device, a device setting stays"
     await gotoPage(desktop, "settings");
     await desktop.getByRole("switch", { name: "Performance mode" }).click();
     await expect(desktop.locator("html")).toHaveAttribute("data-perf", "1");
+    // The Settings prefs save as one payload: the next one (a hints change) would carry performance
+    // mode if it went to the account. Waited for, so the iPad below cannot simply load first.
+    const hints = desktop.getByRole("switch", { name: "Show hints" });
+    const [save] = await Promise.all([savedPrefs(desktop), hints.click()]);
+    expect(Object.keys(save.request().postDataJSON() as object)).not.toContain("performanceMode");
+    await Promise.all([savedPrefs(desktop), hints.click()]);
 
     await openApp(ipad);
     await expect(ipad.locator("html")).not.toHaveAttribute("data-perf", "1");