[test-recorder] Add cloud sign-in guidance for SSO/2FA limitations in recorder Chromium

### Problem
The recorder's Chromium is a separate binary/profile, so Google SSO can be rejected and users get forced into 2FA — a major drop-off observed in live user testing. Session persistence exists (`src/recorder/template.ts:17-23,137-155`) and manual sign-in before recording is possible (`src/recorder/template.ts:163-167`), but no copy warns about SSO/2FA behavior or explains that later sessions reuse the saved login.

### Suggested fix
Add cloud-login guidance at recorder launch: expect SSO to possibly fail, use email+password or 2FA once, sign in *before* pressing Record, and note that the login is remembered for future recordings. Investigate whether a test-account credential flow could bypass personal logins entirely.

_Line references are against branch `cb/comfy-test-usability` (PR #15837); adjust after merge._