fix(#410): align BetterAuth basePath and auth client with NestJS routing

BetterAuth defaulted basePath to /api/auth but NestJS controller routes
to /auth/* (no global prefix). The auth client also pointed at the web
frontend origin instead of the API server, and LoginButton used a
nonexistent GET /auth/signin/authentik endpoint.

- Set basePath: "/auth" in BetterAuth server config
- Point auth client baseURL to API_BASE_URL with matching basePath
- Add genericOAuthClient plugin to auth client
- Use signIn.oauth2({ providerId: "authentik" }) in LoginButton

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

apps/web/src/components/auth/LoginButton.test.tsx

@@ -3,20 +3,19 @@ import { render, screen } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import { LoginButton } from "./LoginButton";
 
-// Mock window.location
-const mockLocation = {
-  href: "",
-  assign: vi.fn(),
-};
-Object.defineProperty(window, "location", {
-  value: mockLocation,
-  writable: true,
-});
+const { mockOAuth2 } = vi.hoisted(() => ({
+  mockOAuth2: vi.fn(),
+}));
+
+vi.mock("@/lib/auth-client", () => ({
+  signIn: {
+    oauth2: mockOAuth2,
+  },
+}));
 
 describe("LoginButton", (): void => {
   beforeEach((): void => {
-    mockLocation.href = "";
-    mockLocation.assign.mockClear();
+    mockOAuth2.mockClear();
   });
 
   it("should render sign in button", (): void => {
@@ -25,14 +24,17 @@ describe("LoginButton", (): void => {
     expect(button).toBeInTheDocument();
   });
 
-  it("should redirect to OIDC endpoint on click", async (): Promise<void> => {
+  it("should initiate OAuth2 sign-in on click", async (): Promise<void> => {
     const user = userEvent.setup();
     render(<LoginButton />);
 
     const button = screen.getByRole("button", { name: /sign in/i });
     await user.click(button);
 
-    expect(mockLocation.assign).toHaveBeenCalledWith("http://localhost:3001/auth/signin/authentik");
+    expect(mockOAuth2).toHaveBeenCalledWith({
+      providerId: "authentik",
+      callbackURL: "/",
+    });
   });
 
   it("should have proper styling", (): void => {

apps/web/src/components/auth/LoginButton.tsx

@@ -1,13 +1,13 @@
 "use client";
 
 import { Button } from "@mosaic/ui";
-import { API_BASE_URL } from "@/lib/config";
+import { signIn } from "@/lib/auth-client";
 
 export function LoginButton(): React.JSX.Element {
   const handleLogin = (): void => {
-    // Redirect to the backend OIDC authentication endpoint
-    // BetterAuth will handle the OIDC flow and redirect back to the callback
-    window.location.assign(`${API_BASE_URL}/auth/signin/authentik`);
+    // Use BetterAuth's genericOAuth client to initiate the OIDC flow.
+    // This POSTs to /auth/sign-in/oauth2 and follows the returned redirect URL.
+    void signIn.oauth2({ providerId: "authentik", callbackURL: "/" });
   };
 
   return (

apps/web/src/lib/auth-client.ts

@@ -7,20 +7,16 @@
  * - Automatic token refresh
  */
 import { createAuthClient } from "better-auth/react";
-// Note: Credentials plugin import removed - better-auth has built-in credentials support
+import { genericOAuthClient } from "better-auth/client/plugins";
+import { API_BASE_URL } from "./config";
 
 /**
- * Auth client instance configured for Jarvis.
+ * Auth client instance configured for Mosaic Stack.
  */
 export const authClient = createAuthClient({
-  // Base URL for auth API
-  baseURL:
-    typeof window !== "undefined"
-      ? window.location.origin
-      : (process.env.BETTER_AUTH_URL ?? "http://localhost:3042"),
-
-  // Plugins can be added here when needed
-  plugins: [],
+  baseURL: API_BASE_URL,
+  basePath: "/auth",
+  plugins: [genericOAuthClient()],
 });
 
 /**
@@ -36,12 +32,7 @@ export const { signIn, signOut, useSession, getSession } = authClient;
  * and the default BetterAuth client expects email.
  */
 export async function signInWithCredentials(username: string, password: string): Promise<unknown> {
-  const baseURL =
-    typeof window !== "undefined"
-      ? window.location.origin
-      : (process.env.BETTER_AUTH_URL ?? "http://localhost:3042");
-
-  const response = await fetch(`${baseURL}/api/auth/sign-in/credentials`, {
+  const response = await fetch(`${API_BASE_URL}/auth/sign-in/credentials`, {
     method: "POST",
     headers: {
       "Content-Type": "application/json",