test(frontend): verify OIDC-only auto-redirect on the login page

Covers the four-way guard added in e5f8610d: redirects when OIDC is the
sole login method, and stays on the form/setup screen when password
login is still enabled, the IdP just bounced with ?error=, or the
server hasn't been set up yet.

frontend/src/routes/login/page.test.ts

@@ -1,4 +1,4 @@
-import { it, expect, vi, beforeEach } from 'vitest';
+import { it, expect, vi, beforeEach, afterEach } from 'vitest';
 import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
 
 const { goto, pageState, session } = vi.hoisted(() => {
@@ -53,6 +53,24 @@ beforeEach(() => {
 	m(auth.getAuthStatus).mockResolvedValue({ initialized: true });
 });
 
+// jsdom's `Location` can't be spied on in place (its setters trigger
+// "not implemented" navigation errors), so swap the whole object for a
+// stub around each test that needs to observe `window.location.replace`.
+const originalLocation = window.location;
+let replaceSpy: ReturnType<typeof vi.fn>;
+
+beforeEach(() => {
+	replaceSpy = vi.fn();
+	Object.defineProperty(window, 'location', {
+		configurable: true,
+		value: { ...originalLocation, replace: replaceSpy }
+	});
+});
+
+afterEach(() => {
+	Object.defineProperty(window, 'location', { configurable: true, value: originalLocation });
+});
+
 it('logs in and redirects', async () => {
 	m(auth.login).mockResolvedValue({ user: { id: '1' } });
 	render(LoginPage);
@@ -179,3 +197,48 @@ it('renders an SSO sign-in link when an OIDC provider is configured', async () =
 	const sso = await screen.findByTestId('login-oidc-btn');
 	expect(sso.getAttribute('href')).toBe('https://idp.test/auth');
 });
+
+it('auto-redirects to the IdP when OIDC is the only login method', async () => {
+	m(auth.getOidcProviders).mockResolvedValue({
+		enabled: true,
+		password_login_enabled: false,
+		authorize_endpoint: '/api/auth/oidc/authorize'
+	});
+	render(LoginPage);
+	await waitFor(() => expect(replaceSpy).toHaveBeenCalledWith('/api/auth/oidc/authorize'));
+});
+
+it('does not auto-redirect when password login is also enabled', async () => {
+	m(auth.getOidcProviders).mockResolvedValue({
+		enabled: true,
+		password_login_enabled: true,
+		authorize_endpoint: '/api/auth/oidc/authorize'
+	});
+	render(LoginPage);
+	await screen.findByTestId('login-form');
+	expect(replaceSpy).not.toHaveBeenCalled();
+});
+
+it('does not auto-redirect after the IdP already returned an error (loop guard)', async () => {
+	pageState.url = new URL('http://localhost/login?error=access_denied');
+	m(auth.getOidcProviders).mockResolvedValue({
+		enabled: true,
+		password_login_enabled: false,
+		authorize_endpoint: '/api/auth/oidc/authorize'
+	});
+	render(LoginPage);
+	await screen.findByTestId('login-form');
+	expect(replaceSpy).not.toHaveBeenCalled();
+});
+
+it('does not auto-redirect during first-run setup', async () => {
+	m(auth.getAuthStatus).mockResolvedValue({ initialized: false });
+	m(auth.getOidcProviders).mockResolvedValue({
+		enabled: true,
+		password_login_enabled: false,
+		authorize_endpoint: '/api/auth/oidc/authorize'
+	});
+	render(LoginPage);
+	await screen.findByTestId('login-setup-form');
+	expect(replaceSpy).not.toHaveBeenCalled();
+});