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/components/pos/pos-register.tsx

@@ -0,0 +1,72 @@
+import { useEffect } from 'react'
+import { useQuery, useQueryClient } from '@tanstack/react-query'
+import { queryOptions } from '@tanstack/react-query'
+import { api } from '@/lib/api-client'
+import { usePOSStore } from '@/stores/pos.store'
+import { currentDrawerOptions, transactionOptions } from '@/api/pos'
+import { POSTopBar } from './pos-top-bar'
+import { POSItemPanel } from './pos-item-panel'
+import { POSCartPanel } from './pos-cart-panel'
+
+interface Location {
+  id: string
+  name: string
+}
+
+function locationsOptions() {
+  return queryOptions({
+    queryKey: ['locations'],
+    queryFn: () => api.get<{ data: Location[] }>('/v1/locations'),
+  })
+}
+
+export function POSRegister() {
+  const { locationId, setLocation, currentTransactionId, drawerSessionId, setDrawerSession } = usePOSStore()
+  const queryClient = useQueryClient()
+
+  // Fetch locations
+  const { data: locationsData } = useQuery(locationsOptions())
+  const locations = locationsData?.data ?? []
+
+  // Auto-select first location
+  useEffect(() => {
+    if (!locationId && locations.length > 0) {
+      setLocation(locations[0].id)
+    }
+  }, [locationId, locations, setLocation])
+
+  // Fetch current drawer for selected location
+  const { data: drawer } = useQuery({
+    ...currentDrawerOptions(locationId),
+    retry: false,
+  })
+
+  // Sync drawer session ID
+  useEffect(() => {
+    if (drawer?.id && drawer.status === 'open') {
+      setDrawerSession(drawer.id)
+    }
+  }, [drawer, setDrawerSession])
+
+  // Fetch current transaction
+  const { data: transaction } = useQuery(transactionOptions(currentTransactionId))
+
+  return (
+    <div className="flex flex-col h-full">
+      <POSTopBar
+        locations={locations}
+        locationId={locationId}
+        onLocationChange={setLocation}
+        drawer={drawer ?? null}
+      />
+      <div className="flex flex-1 min-h-0">
+        <div className="w-[60%] border-r border-border overflow-hidden">
+          <POSItemPanel transaction={transaction ?? null} />
+        </div>
+        <div className="w-[40%] overflow-hidden">
+          <POSCartPanel transaction={transaction ?? null} />
+        </div>
+      </div>
+    </div>
+  )
+}