Fix auth when serving Frigate at a subpath (#12815)

Ensure axios.defaults.baseURL is set when accessing login form.

Drop `/api` prefix in login form's `axios.post` call, since `/api` is
part of the baseURL.

Redirect to subpath on succesful authentication.

Prepend subpath to default logout url.

Fixes #12814

web/src/login.tsx

@@ -1,6 +1,7 @@
 import React from "react";
 import ReactDOM from "react-dom/client";
 import LoginPage from "@/pages/LoginPage.tsx";
+import "@/api";
 import "./index.css";
 
 ReactDOM.createRoot(document.getElementById("root")!).render(