feat(integrations): add OpenAI API settings

[darwin67]

Summary

• add a shared responsive settings shell and restyle account credential forms
• add authenticated OpenAI Platform API-key connect, replace, and disconnect flows with fresh sudo checks
• validate saved keys asynchronously against the fixed metadata endpoint with redirect rejection and generation fencing
• record credential-free lifecycle audit events and link provider-side key revocation guidance

Security boundaries

• keys are submitted without phx-change, encrypted before persistence, and never retained in LiveView assigns
• validation uses a fixed HTTPS origin, disables redirects and retries, and persists only bounded outcomes
• /integrations remains in the existing authenticated browser pipeline and :require_authenticated_user LiveView session

Verification

• mix precommit (398 passed, 5 excluded; Credo clean)
• desktop and 390px mobile browser checks