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/stores/pos.store.ts

@@ -0,0 +1,21 @@
+import { create } from 'zustand'
+
+interface POSState {
+  currentTransactionId: string | null
+  locationId: string | null
+  drawerSessionId: string | null
+  setTransaction: (id: string | null) => void
+  setLocation: (id: string) => void
+  setDrawerSession: (id: string | null) => void
+  reset: () => void
+}
+
+export const usePOSStore = create<POSState>((set) => ({
+  currentTransactionId: null,
+  locationId: null,
+  drawerSessionId: null,
+  setTransaction: (id) => set({ currentTransactionId: id }),
+  setLocation: (id) => set({ locationId: id }),
+  setDrawerSession: (id) => set({ drawerSessionId: id }),
+  reset: () => set({ currentTransactionId: null }),
+}))