refactor: move oidc handling to backend and add support for oidc post (#923)

Co-authored-by: Claude <noreply@anthropic.com>

frontend/src/lib/hooks/login-for.ts

@@ -0,0 +1,17 @@
+type UseLoginForProps = {
+  login_for?: "oidc" | "app";
+  compiledParams: string;
+};
+
+export const useLoginFor = (props: UseLoginForProps): string => {
+  const { login_for, compiledParams } = props;
+
+  switch (login_for) {
+    case "oidc":
+      return "/oidc/authorize" + compiledParams;
+    case "app":
+      return "/continue" + compiledParams;
+    default:
+      return "/logout";
+  }
+};