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/routes/pos.tsx

@@ -0,0 +1,21 @@
+import { createFileRoute, redirect } from '@tanstack/react-router'
+import { useAuthStore } from '@/stores/auth.store'
+import { POSRegister } from '@/components/pos/pos-register'
+
+export const Route = createFileRoute('/pos')({
+  beforeLoad: () => {
+    const { token } = useAuthStore.getState()
+    if (!token) {
+      throw redirect({ to: '/login' })
+    }
+  },
+  component: POSPage,
+})
+
+function POSPage() {
+  return (
+    <div className="h-screen w-screen overflow-hidden bg-background text-foreground">
+      <POSRegister />
+    </div>
+  )
+}