feat(web): use Better-Auth session state in App.tsx (GRO-126)

Add useSession hook to check Better-Auth session for production auth.
Redirect to Authentik sign-in when no session in production mode.
Dev mode flow (DevLoginSelector) preserved.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/App.tsx

@@ -17,6 +17,7 @@ import { DevLoginSelector, getDevUser } from "./pages/DevLoginSelector.js";
 import { DevSessionIndicator } from "./components/DevSessionIndicator.js";
 import { BrandingProvider, useBranding } from "./BrandingContext.js";
 import { GlobalSearch } from "./components/GlobalSearch.js";
+import { useSession, signIn } from "./lib/auth-client.js";
 
 const NAV_LINKS = [
   { to: "/admin", label: "Appointments" },
@@ -133,6 +134,7 @@ function AdminLayout() {
 export function App() {
   const location = useLocation();
   const [authDisabled, setAuthDisabled] = useState<boolean | null>(null);
+  const { data: session, isPending: sessionLoading } = useSession();
 
   useEffect(() => {
     fetch("/api/dev/config")
@@ -141,19 +143,11 @@ export function App() {
       .catch(() => setAuthDisabled(false));
   }, []);
 
-  // Show login selector page
-  if (location.pathname === "/login") {
+  // Show login selector page (only in development)
+  if (import.meta.env.DEV && location.pathname === "/login") {
     return <DevLoginSelector />;
   }
 
-  // While checking auth config, render nothing briefly
-  if (authDisabled === null) return null;
-
-  // If auth is disabled and no dev user is selected, redirect to login selector
-  if (authDisabled && !getDevUser() && location.pathname !== "/login") {
-    return <Navigate to="/login" replace />;
-  }
-
   // Public booking redirect pages — no auth or portal chrome needed
   if (location.pathname === "/booking/confirmed") {
     return <BookingConfirmedPage />;
@@ -165,6 +159,20 @@ export function App() {
     return <BookingErrorPage />;
   }
 
+  // Still loading auth state
+  if (authDisabled === null || sessionLoading) return null;
+
+  // Dev mode: use dev login selector
+  if (authDisabled && !getDevUser() && location.pathname !== "/login") {
+    return <Navigate to="/login" replace />;
+  }
+
+  // Production mode: if no session, redirect to Authentik sign-in
+  if (!authDisabled && !session) {
+    signIn.social({ provider: "authentik" });
+    return null;
+  }
+
   return (
     <BrandingProvider>
       {location.pathname.startsWith("/admin") ? (