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/app/routes/me.py

@@ -136,7 +136,10 @@ def update_preferences(
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    # Merge so partial updates don't wipe other keys.
+    # Merge so partial updates don't wipe other keys — over the stored row read again under a row
+    # lock: a client saves keys one at a time (several at once on a boot that moves this browser's
+    # copies to the account), and two merges over the same earlier read would keep only the last.
+    db.refresh(user, with_for_update=True)
     merged = dict(user.preferences or {})
     merged.update(preferences.root)
     user.preferences = merged