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/global.setup.ts

@@ -12,7 +12,8 @@ import { retryConnect } from "./retryConnect";
  *   2. seed — reset the dedicated E2E accounts to a known baseline, in-container (idempotent);
  *   3. authenticate each ONCE via password-login and persist its session + `en` locale to its own
  *      storageState, so the real specs never re-auth (auth endpoints are rate-limited). The suite
- *      runs as the admin; `member.spec.ts` runs as the plain member, from MEMBER_STORAGE.
+ *      runs as the admin; `member.spec.ts` and `devices.spec.ts` run as the plain member, from
+ *      MEMBER_STORAGE.
  */
 
 const STORAGE = "e2e/.auth/state.json";