IAV-1362/feat(telephony): migrate campaigns to PhoneAssistant, add scheduling and prospect management
- Replace PhoneAgent with PhoneAssistant on campaigns (assistantUid/assistantName)
- Add scheduledStartAt with date/time pickers (MUI X, 10-min slots)
- Campaign creation: save as draft / schedule / launch now actions
- Allow reverting PLANNED campaign back to DRAFT
- Allow deleting DRAFT and PLANNED campaigns
- Add prospect management on non-ended campaigns (add/remove uncalled)
- Block assistant deletion when used by active campaign (soft-delete otherwise)
- Fix campaign table column alignment and remove horizontal scrollbar
- Add DELETED status to TelephonyAgentStatus enum
- Add revertToDraft, addProspects, removeProspect store actions and endpoints
Closes IAV-1362