feat(auth): Complete OIDC security refactor and modernize test suite

- Refactored OIDC flow to implement PKCE, state/nonce validation, and BFF pattern.
- Centralized configuration in Settings class (DEV_MODE, FRONTEND_URL, OIDC_REDIRECT_URI).
- Updated auth routers to use conditional secure cookie flags based on DEV_MODE.
- Modernized and cleaned up test suite by removing legacy Streamlit tests.
- Fixed linting errors and unused imports across the backend.

frontend/src/components/auth/AuthCallback.tsx

@@ -1,39 +0,0 @@
-import { useEffect } from "react"
-import { useNavigate } from "react-router-dom"
-import { AuthService } from "@/services/auth"
-
-export function AuthCallback() {
-  const navigate = useNavigate()
-
-  useEffect(() => {
-    const verifyAuth = async () => {
-      const urlParams = new URLSearchParams(window.location.search)
-      const code = urlParams.get("code")
-
-      try {
-        if (code) {
-          // If we have a code, exchange it for a cookie
-          await AuthService.exchangeOIDCCode(code)
-        } else {
-          // If no code, just verify existing cookie (backend-driven redirect)
-          await AuthService.getMe()
-        }
-        
-        // Success - go to home. We use window.location.href to ensure a clean reload of App state
-        window.location.href = "/"
-      } catch (err) {
-        console.error("Auth callback verification failed:", err)
-        navigate("/?error=auth_failed", { replace: true })
-      }
-    }
-
-    verifyAuth()
-  }, [navigate])
-
-  return (
-    <div className="min-h-screen flex flex-col items-center justify-center bg-background">
-      <div className="animate-spin rounded-full h-8 w-8 border-b-2 border-primary mb-4"></div>
-      <p className="text-muted-foreground">Completing login...</p>
-    </div>
-  )
-}

frontend/src/components/auth/LoginForm.tsx

@@ -22,10 +22,11 @@ import axios from "axios"
 interface LoginFormProps {
   onSuccess: () => void
   onToggleMode: () => void
+  externalError?: string | null
 }
 
-export function LoginForm({ onSuccess, onToggleMode }: LoginFormProps) {
-  const [error, setError] = useState<string | null>(null)
+export function LoginForm({ onSuccess, onToggleMode, externalError }: LoginFormProps) {
+  const [error, setError] = useState<string | null>(externalError || null)
   const [isLoading, setIsLoading] = useState(false)
 
   const form = useForm<LoginInput>({