feat: AI chat system with LangGraph, multi-step patient flows, and appointment rescheduling

- Add floating chat window Hand-off to AI toggle (per-patient) and after-hours AI toggle (global)
- Add LangGraph-powered appointment reminder flow: AI introduces itself, classifies YES/NO, handles confirmation with appointment date/time
- Add multi-step rescheduling flow: ASAP vs next week, tomorrow offer, Mon/Tue/Wed picker, morning/afternoon time slot — automatically updates appointment in DB
- Add new patient / after-hours flow: new vs existing patient, dental insurance check, MassHealth Selenium eligibility check (auto-uses saved member ID + DOB for existing patients), self-pay fallback
- Add AI Chat Settings page (Settings → Advanced) with editable greeting templates and LangGraph flow diagrams for both reminder and new-patient flows
- Add Schedule a New Patient template option in chat window, starts new-patient conversation flow
- Add GET/PUT endpoints for AI handoff, after-hours handoff, and AI chat templates
- Add multilingual support (7 languages) across all AI reply nodes with LLM generation and hardcoded fallbacks
- Add pending reschedule in-memory store and conversation stage tracking across all flows

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/Frontend/src/pages/settings-page.tsx

@@ -19,6 +19,7 @@ import { OfficeHoursCard } from "@/components/settings/office-hours-card";
 import { OfficeContactCard } from "@/components/settings/office-contact-card";
 import { ProcedureTimeslotCard } from "@/components/settings/procedure-timeslot-card";
 import { InsuranceContactCard } from "@/components/settings/insurance-contact-card";
+import { AiChatSettingsCard } from "@/components/settings/ai-chat-settings-card";
 
 type SectionId =
   | "staff"
@@ -29,6 +30,7 @@ type SectionId =
   | "programs"
   | "twilio"
   | "ai"
+  | "aichat"
   | "officehours"
   | "officecontact"
   | "proceduretimeslot"
@@ -259,6 +261,9 @@ export default function SettingsPage() {
       case "ai":
         return <AiSettingsCard />;
 
+      case "aichat":
+        return <AiChatSettingsCard />;
+
       case "officehours":
         return <OfficeHoursCard />;