refactor(frontend): one account per browser, its local state under plain keys

The per-tab account pin, the X-Siftlode-Account header and ?account= go: the session cookie alone
says who is asking. localStorage keys lose their account suffix; the browser records whose state it
holds (siftlode.owner), BootGate hands it to the account the server answered as, and a different
account starts from a clean slate (the public /watch keys stay). The first boot moves the account's
old per-account copies onto the plain keys and drops other accounts' copies. Until the claim, and
after another tab claimed the browser for someone else, the account helpers read and write nothing.

The account panel lists no other accounts: "Add another account" becomes "Sign in as someone else"
(Google's account chooser; the pick replaces this account, a cancel keeps it), shown where Google
sign-in is configured.

frontend/src/App.tsx

@@ -270,8 +270,8 @@ export default function App() {
     clearParams("link");
   }, [t, setPage, refetchMe]);
 
-  // A Google OAuth flow started while already signed in (add-another-account, link, or upgrade) was
-  // cancelled at Google's consent screen. The backend only emits this for a live session, so a
+  // A Google OAuth flow started while already signed in (signing in as someone else, or a Google
+  // connect) was cancelled at Google's consent screen. The backend only emits this for a live session, so a
   // neutral toast is the right feedback — the pre-auth login banner would never render here.
   // Same one-shot shape as the `?link=` handler above, same reason for the ref.
   const oauthAnswered = useRef(false);