Milestone 3: catalog sync, batch transactions, and reports

- GET /api/v1/catalog/sync?since= — delta sync for Android offline-first
- POST /api/v1/transactions/batch — idempotency-keyed batch upload (207 Multi-Status),
  validates product ownership, skips duplicates silently
- GET /api/v1/transactions + /reports/summary — paginated list and aggregated
  revenue/tax/top-product reporting with date range filters
- ReportsPage: stat cards, payment method breakdown, top-10 products, transaction table
- Reports added to sidebar nav and router

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/App.tsx

@@ -6,6 +6,7 @@ import DashboardPage from "./pages/DashboardPage";
 import UsersPage from "./pages/UsersPage";
 import CatalogPage from "./pages/CatalogPage";
 import VendorPage from "./pages/VendorPage";
+import ReportsPage from "./pages/ReportsPage";
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const { user, loading } = useAuth();
@@ -44,6 +45,7 @@ export default function App() {
           <Route path="catalog" element={<CatalogPage />} />
           <Route path="users" element={<UsersPage />} />
           <Route path="vendor" element={<VendorPage />} />
+          <Route path="reports" element={<ReportsPage />} />
           <Route path="*" element={<Navigate to="/" replace />} />
         </Route>
       </Routes>