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/Backend/src/ai/aiHandoffStore.ts

@@ -0,0 +1,95 @@
+// In-memory store for per-patient AI handoff toggle, conversation stage,
+// and per-user after-hours handoff toggle.
+// Conversation key: `${userId}:${patientId}`
+
+export type ConversationStage =
+  | "initial"                       // default — no active conversation
+  | "reminder_initial"              // office sent a reminder, waiting for first patient reply
+  | "greeted"                       // reminder intro sent, waiting for yes/no
+  | "done"                          // conversation complete
+  | "new_patient_greeted"           // new-patient greeting sent, waiting for patient intent
+  | "asked_new_or_existing"         // AI asked "new or existing patient?"
+  | "asked_new_patient_insurance"   // AI asked new patient about insurance
+  | "asked_existing_insurance"      // AI asked existing patient about same insurance
+  | "asked_appointment_time"        // AI asked when they'd like to come
+  | "awaiting_masshealth_info"      // AI asked for Member ID + DOB, waiting for reply
+  | "asked_appointment_preference"  // Selenium: ACTIVE — AI asked check-up vs problem
+  | "asked_self_pay"                // Selenium: INACTIVE — AI asked if self-pay exam
+  | "asked_reschedule_confirm"      // AI asked "Would you like to reschedule?"
+  | "asked_reschedule_preference"   // AI asked ASAP vs next week
+  | "asked_reschedule_asap"         // AI asked "Can you come tomorrow?"
+  | "asked_reschedule_next_week"    // AI offered Mon/Tue/Wed next week
+  | "asked_reschedule_time";        // Day confirmed — AI asked morning or afternoon
+
+const handoffStore     = new Map<string, boolean>();
+const stageStore       = new Map<string, ConversationStage>();
+const afterHoursStore  = new Map<number, boolean>(); // keyed by userId
+
+function convKey(userId: number, patientId: number): string {
+  return `${userId}:${patientId}`;
+}
+
+// ── Per-patient handoff toggle (default ON) ───────────────────────────────────
+
+export function getHandoff(userId: number, patientId: number): boolean {
+  const k = convKey(userId, patientId);
+  return handoffStore.has(k) ? handoffStore.get(k)! : true;
+}
+
+export function setHandoff(userId: number, patientId: number, enabled: boolean): void {
+  handoffStore.set(convKey(userId, patientId), enabled);
+}
+
+// ── Per-user after-hours handoff toggle (default ON) ─────────────────────────
+
+export function getAfterHoursHandoff(userId: number): boolean {
+  return afterHoursStore.has(userId) ? afterHoursStore.get(userId)! : true;
+}
+
+export function setAfterHoursHandoff(userId: number, enabled: boolean): void {
+  afterHoursStore.set(userId, enabled);
+}
+
+// ── Conversation stage ────────────────────────────────────────────────────────
+
+export function getStage(userId: number, patientId: number): ConversationStage {
+  return stageStore.get(convKey(userId, patientId)) ?? "initial";
+}
+
+export function setStage(userId: number, patientId: number, stage: ConversationStage): void {
+  stageStore.set(convKey(userId, patientId), stage);
+}
+
+// Called when office sends an outbound reminder — marks next patient reply
+// as the start of a reminder conversation.
+export function resetConversation(userId: number, patientId: number): void {
+  stageStore.set(convKey(userId, patientId), "reminder_initial");
+}
+
+// Called when office sends the new-patient greeting — marks next patient reply
+// as the start of the new-patient conversation flow.
+export function startNewPatientConversation(userId: number, patientId: number): void {
+  stageStore.set(convKey(userId, patientId), "new_patient_greeted");
+}
+
+// ── Pending reschedule data ───────────────────────────────────────────────────
+// Holds the confirmed new date while AI waits for a time-slot answer.
+
+interface PendingReschedule {
+  newDate:  Date;   // JS Date for the new appointment day (midnight UTC)
+  dayLabel: string; // human-readable, e.g. "Tuesday, May 19"
+}
+
+const pendingRescheduleStore = new Map<string, PendingReschedule>();
+
+export function setPendingReschedule(userId: number, patientId: number, data: PendingReschedule): void {
+  pendingRescheduleStore.set(convKey(userId, patientId), data);
+}
+
+export function getPendingReschedule(userId: number, patientId: number): PendingReschedule | undefined {
+  return pendingRescheduleStore.get(convKey(userId, patientId));
+}
+
+export function clearPendingReschedule(userId: number, patientId: number): void {
+  pendingRescheduleStore.delete(convKey(userId, patientId));
+}