chore(web): guard root element instead of non-null assertion

apps/web/src/main.tsx

@@ -12,7 +12,12 @@ initAnalytics()
 
 const queryClient = new QueryClient()
 
-createRoot(document.getElementById("root")!).render(
+const rootElement = document.getElementById("root")
+if (!rootElement) {
+  throw new Error('Élément racine "#root" introuvable')
+}
+
+createRoot(rootElement).render(
   <StrictMode>
     <QueryClientProvider client={queryClient}>
       <ThemeProvider>