Align provider buttons and Google iframe name

Found while testing Task 8579 on /login and /signup (local stack and app.jaah.ai). - The provider buttons (GitHub, Microsoft, SSO) sit 10px in from each side, matching GSI's iframe chrome, so they are narrower than the email/password fields and the Sign in / Continue button right above or below them. Line up their edges with the form column, or make the whole column one width. - On /signup, Chrome names the Google iframe "Sign in with Google Button", which contradicts the page's "Sign up with GitHub/Microsoft" labels. Give it a sign-up-specific name (GSI `text: 'signup_with'` and/or an iframe `title`) so every provider reads as one set. - A present-but-expired session cookie still makes /login log a 401 for `GET /api/auth/me` (Task 8579 handled only a visit with no cookie at all). Decide whether an expired cookie should also get a quiet answer, for example by clearing the cookie on logout or expiry so the next visit sends none. Out of scope: which providers are offered, and how they are gated (Task 8567). Done means: screenshots at 390px and 1280px show the provider buttons flush with the form controls; a Playwright accessibility snapshot of /signup names every provider control "Sign up with …"; the expired-cookie case is decided and its behaviour is covered by a test.

Provider buttons now flush with the form on /login and /signup; Google iframe named "Sign up with Google"; /me probe answers 204 and clears a dead session cookie (HIPAA idle keeps 401). Tests and quick gate green; review lane passed; #6513 merged, Jaah Deploy succeeded. Follow-up draft 8657.