IAV-1309: Assistant integrations panel, phone number edit form & UX polish

Assistant (assistant-section.tsx)

• Replace TelephonyCalendarIntegrationsPanel with a self-contained AssistantIntegrationsPanel using exclusive/radio behavior (only one calendar integration active at a time)
• Data loading aligned with IntegrationsSection (wizard status + all agents fetched in parallel)
• Agent detail dialog: row 2 now spans full width across 3 equal columns
• Voice column in table and detail dialog now displays voice ID (e.g. nova, ash, alloy)

Integrations (integrations-section.tsx)

• Allow up to 2 integrations active simultaneously (removed radio deactivation)
• Enforce minimum 1 active integration — block deactivation attempt with error snackbar

Phone Numbers (phone-numbers-section.tsx)

• Edit number dialog restructured to mirror the creation form (phone info + provider sections) with an added status field
• Deactivation confirmation dialog uses agent brand color instead of yellow