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

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

frontend/src/lib/hooks/screen-params.ts

@@ -0,0 +1,40 @@
+import { z } from "zod";
+
+type ScreenParams = {
+  login_for?: "oidc" | "app";
+  redirect_uri?: string;
+  oidc_ticket?: string;
+  oidc_scope?: string;
+  oidc_name?: string;
+};
+
+const zodScreenParams = z.object({
+  login_for: z.enum(["oidc", "app"]).optional(),
+  redirect_uri: z.string().optional(),
+  oidc_ticket: z.string().optional(),
+  oidc_scope: z.string().optional(),
+  oidc_name: z.string().optional(),
+});
+
+export function useScreenParams(params: URLSearchParams): ScreenParams {
+  const paramsObj = Object.fromEntries(params.entries());
+  const parsed = zodScreenParams.safeParse(paramsObj);
+  if (!parsed.success) {
+    return {};
+  }
+  return parsed.data;
+}
+
+export function recompileScreenParams(params: ScreenParams): string {
+  const p = new URLSearchParams(
+    Object.fromEntries(
+      Object.entries(params).filter(([, v]) => v !== undefined),
+    ) as Record<string, string>,
+  ).toString();
+
+  if (p.length > 0) {
+    return "?" + p;
+  }
+
+  return "";
+}