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.

backend/tests/conftest.py

@@ -231,7 +231,7 @@ def _configured(monkeypatch):
 @pytest.fixture
 def client(db, override_dep, _configured):
     """A TestClient wired to the test DB that goes through the REAL auth/session stack — no
-    `current_user` override — so login, the wallet gate and epoch revoke are exercised end to end.
+    `current_user` override — so login, the one-account session and epoch revoke are exercised end to end.
     Starlette's SessionMiddleware round-trips the signed cookie, so a login in one call authenticates
     the next on this same instance. No lifespan (the scheduler / HLS sweep stay off)."""
     override_dep(get_db, db.info["request"])