Fix OAuth: Add baseUrl getter to ApiClient

- OAuth login URLs need baseUrl without /api/v1 suffix
- Added getter that derives baseUrl from getApiBaseUrl()

frontend/src/lib/api.ts

@@ -46,6 +46,10 @@ interface ApiError {
 
 class ApiClient {
   private token: string | null = null
+  
+  get baseUrl(): string {
+    return getApiBaseUrl().replace('/api/v1', '')
+  }
 
   setToken(token: string | null) {
     this.token = token