Phase 1b — Buffer settings UI

Phase 1b of Kazokus Social Publishing — Settings → Integrations → Buffer UI. Customer admins can paste a Buffer personal access token, see connection state (org, plan, channels), toggle "require second approver", and disconnect. Consumes the backend API shipped in #549 + fixed in #550.

Plan: docs/superpowers/plans/2026-04-17-kazokus-social-publishing.md (Phase 1b section, Tasks 12-14).