IAV-1311/ sub-agents coming soon + enforce min integration per family

• Sub-agents section (SAV): marked as comingSoon — header is grayed out, list area replaced by a "Bientôt disponible" placeholder. The comingSoon prop on SubAgentsSection controls this so it can be re-enabled with a one-line change.

• Wizard — step integrations: added a guard (handleToggleWithGuard) that intercepts deselection of the last integration in a family and shows a warning snackbar instead of silently blocking the action (previous behavior was a no-op in handleIntegrationToggle).

• SAV dashboard — integration section: integrations are now grouped by family (Canaux de communication / Systèmes de ticketing / E-commerce) with a colored left-border label. handlePlatformToggle now blocks deactivation of the last connected+enabled integration per family and surfaces a snackbar error.