SSO login fix (#5167)

Fixes bug where SSO login with custom providers caused an
`InvalidClientRegistrationIdException: Invalid Client Registration with
Id: oidc` errors.

  Root Cause:
- Backend: Redirect URI was hardcoded to `/login/oauth2/code/oidc`
regardless of provider registration ID
- Frontend: Unknown providers were mapped back to 'oidc' instead of
using actual provider ID

Closes #5141

---------

Co-authored-by: Anthony Stirling <[email protected]>
Co-authored-by: Keon Chen <[email protected]>

frontend/src/proprietary/auth/oauthTypes.ts

@@ -0,0 +1,24 @@
+/**
+ * Known OAuth providers with dedicated UI support.
+ * Custom providers are also supported - the backend determines availability.
+ */
+export const KNOWN_OAUTH_PROVIDERS = [
+  'github',
+  'google',
+  'apple',
+  'azure',
+  'keycloak',
+  'cloudron',
+  'authentik',
+  'oidc',
+] as const;
+
+export type KnownOAuthProvider = typeof KNOWN_OAUTH_PROVIDERS[number];
+
+/**
+ * OAuth provider ID - can be any known provider or custom string.
+ * The backend configuration determines which providers are available.
+ *
+ * @example 'github' | 'google' | 'mycompany' | 'authentik'
+ */
+export type OAuthProvider = KnownOAuthProvider | (string & {});

frontend/src/proprietary/auth/springAuthClient.ts

@@ -10,6 +10,7 @@
 import apiClient from '@app/services/apiClient';
 import { AxiosError } from 'axios';
 import { BASE_PATH } from '@app/constants/app';
+import { type OAuthProvider } from '@app/auth/oauthTypes';
 
 // Helper to extract error message from axios error
 function getErrorMessage(error: unknown, fallback: string): string {
@@ -248,11 +249,14 @@ class SpringAuthClient {
   }
 
   /**
-   * Sign in with OAuth provider (GitHub, Google, etc.)
+   * Sign in with OAuth provider (GitHub, Google, Authentik, etc.)
    * This redirects to the Spring OAuth2 authorization endpoint
+   *
+   * @param params.provider - OAuth provider ID (e.g., 'github', 'google', 'authentik', 'mycompany')
+   *                          Can be any known provider or custom string - the backend determines available providers
    */
   async signInWithOAuth(params: {
-    provider: 'github' | 'google' | 'apple' | 'azure' | 'keycloak' | 'oidc';
+    provider: OAuthProvider;
     options?: { redirectTo?: string; queryParams?: Record<string, any> };
   }): Promise<{ error: AuthError | null }> {
     try {