feat: implement admin and OIDC authentication methods; add login and user info endpoints

Signed-off-by: Noste <83548733+Noooste@users.noreply.github.com>

frontend/src/components/auth/LoadingSpinner.tsx

@@ -0,0 +1,12 @@
+import { Loader2 } from 'lucide-react';
+
+export function LoadingSpinner() {
+  return (
+    <div className="flex h-screen w-full items-center justify-center">
+      <div className="flex flex-col items-center gap-4">
+        <Loader2 className="h-8 w-8 animate-spin text-primary" />
+        <p className="text-sm text-muted-foreground">Loading...</p>
+      </div>
+    </div>
+  );
+}