Add accessible names to GitHub, Microsoft, SSO

Found while testing Task 8567 on app.jaah.ai/signup and /login. - The "Sign up with GitHub" link (`/api/auth/oauth/github/start?intent=signup`) and the "Sign in with SSO" link (`/login?sso=1`) appear in the accessibility tree as links with NO name: screen readers announce just "link". Give each the visible label as its accessible name (the Google button already has one). - The sign-up and login pages log 2 console errors on first paint on prod; find and fix their cause or silence the expected ones. Out of scope: changing which providers are offered or their gating (Task 8567). Done means: a Playwright accessibility snapshot of /signup and /login names every provider link by its label, and both pages load with no console errors.

GitHub/Microsoft/SSO links carry their label as aria-label. A cookieless /login and /signup no longer log 401s: /me and /signup/pending answer 204 to an opt-in probe. Quick gate green, PR #6486 merged (f15eb58bb). The deploy was superseded by a newer one. Follow-up: Task 8589.