feat: add POS register screen with full-screen touch-optimized layout

Standalone register at /pos bypassing the admin sidebar layout:
- Two-panel layout: product search/grid (60%) + cart/payment (40%)
- Product search with barcode scan support (UPC lookup on Enter)
- Custom item entry dialog for ad-hoc items
- Cart with line items, tax, totals, and remove-item support
- Payment dialogs: cash (quick amounts + change calc), card, check
- Drawer open/close with balance reconciliation and over/short
- Auto-creates pending transaction on first item added
- POS link added to admin sidebar nav (module-gated)
- Zustand store for POS session state, React Query for server data

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/admin/src/api/pos.ts

@@ -0,0 +1,163 @@
+import { queryOptions } from '@tanstack/react-query'
+import { api } from '@/lib/api-client'
+
+// --- Types ---
+
+export interface Transaction {
+  id: string
+  locationId: string | null
+  transactionNumber: string
+  accountId: string | null
+  repairTicketId: string | null
+  repairBatchId: string | null
+  transactionType: string
+  status: string
+  subtotal: string
+  discountTotal: string
+  taxTotal: string
+  total: string
+  paymentMethod: string | null
+  amountTendered: string | null
+  changeGiven: string | null
+  checkNumber: string | null
+  roundingAdjustment: string
+  taxExempt: boolean
+  taxExemptReason: string | null
+  processedBy: string
+  drawerSessionId: string | null
+  notes: string | null
+  completedAt: string | null
+  createdAt: string
+  updatedAt: string
+  lineItems?: TransactionLineItem[]
+}
+
+export interface TransactionLineItem {
+  id: string
+  transactionId: string
+  productId: string | null
+  inventoryUnitId: string | null
+  description: string
+  qty: number
+  unitPrice: string
+  discountAmount: string
+  discountReason: string | null
+  taxRate: string
+  taxAmount: string
+  lineTotal: string
+  createdAt: string
+}
+
+export interface DrawerSession {
+  id: string
+  locationId: string | null
+  openedBy: string
+  closedBy: string | null
+  openingBalance: string
+  closingBalance: string | null
+  expectedBalance: string | null
+  overShort: string | null
+  denominations: Record<string, number> | null
+  status: string
+  notes: string | null
+  openedAt: string
+  closedAt: string | null
+}
+
+export interface Discount {
+  id: string
+  name: string
+  discountType: string
+  discountValue: string
+  appliesTo: string
+  requiresApprovalAbove: string | null
+  isActive: boolean
+}
+
+export interface Product {
+  id: string
+  name: string
+  sku: string | null
+  upc: string | null
+  description: string | null
+  sellingPrice: string | null
+  costPrice: string | null
+  qtyOnHand: number | null
+  taxCategory: string
+  isSerialized: boolean
+  isActive: boolean
+}
+
+// --- Query Keys ---
+
+export const posKeys = {
+  transaction: (id: string) => ['pos', 'transaction', id] as const,
+  drawer: (locationId: string) => ['pos', 'drawer', locationId] as const,
+  products: (search: string) => ['pos', 'products', search] as const,
+  discounts: ['pos', 'discounts'] as const,
+}
+
+// --- Query Options ---
+
+export function transactionOptions(id: string | null) {
+  return queryOptions({
+    queryKey: posKeys.transaction(id ?? ''),
+    queryFn: () => api.get<Transaction>(`/v1/transactions/${id}`),
+    enabled: !!id,
+  })
+}
+
+export function currentDrawerOptions(locationId: string | null) {
+  return queryOptions({
+    queryKey: posKeys.drawer(locationId ?? ''),
+    queryFn: () => api.get<DrawerSession>('/v1/drawer/current', { locationId }),
+    enabled: !!locationId,
+    retry: false,
+  })
+}
+
+export function productSearchOptions(search: string) {
+  return queryOptions({
+    queryKey: posKeys.products(search),
+    queryFn: () => api.get<{ data: Product[]; pagination: any }>('/v1/products', { q: search, limit: 24, isActive: true }),
+    enabled: search.length >= 1,
+  })
+}
+
+export function discountListOptions() {
+  return queryOptions({
+    queryKey: posKeys.discounts,
+    queryFn: () => api.get<Discount[]>('/v1/discounts/all'),
+  })
+}
+
+// --- Mutations ---
+
+export const posMutations = {
+  createTransaction: (data: { transactionType: string; locationId?: string }) =>
+    api.post<Transaction>('/v1/transactions', data),
+
+  addLineItem: (txnId: string, data: { productId?: string; inventoryUnitId?: string; description: string; qty: number; unitPrice: number }) =>
+    api.post<TransactionLineItem>(`/v1/transactions/${txnId}/line-items`, data),
+
+  removeLineItem: (txnId: string, lineItemId: string) =>
+    api.del<TransactionLineItem>(`/v1/transactions/${txnId}/line-items/${lineItemId}`),
+
+  applyDiscount: (txnId: string, data: { discountId?: string; amount: number; reason: string; lineItemId?: string }) =>
+    api.post<Transaction>(`/v1/transactions/${txnId}/discounts`, data),
+
+  complete: (txnId: string, data: { paymentMethod: string; amountTendered?: number; checkNumber?: string }) =>
+    api.post<Transaction>(`/v1/transactions/${txnId}/complete`, data),
+
+  void: (txnId: string) =>
+    api.post<Transaction>(`/v1/transactions/${txnId}/void`, {}),
+
+  openDrawer: (data: { locationId?: string; openingBalance: number }) =>
+    api.post<DrawerSession>('/v1/drawer/open', data),
+
+  closeDrawer: (id: string, data: { closingBalance: number; denominations?: Record<string, number>; notes?: string }) =>
+    api.post<DrawerSession>(`/v1/drawer/${id}/close`, data),
+
+  lookupUpc: (upc: string) =>
+    api.get<Product>(`/v1/products/lookup/upc/${upc}`),
+}