refactor: migrate to Next.js with edge functions and update relay utilities

- Removed old relay utilities and tests, replacing them with a new structure under `src/lib/relay-utils.ts`.
- Implemented a new edge function in `app/api/proxy/route.ts` to handle proxy requests.
- Created a new Next.js layout and page structure for the frontend, including a user interface for testing the proxy.
- Added Tailwind CSS for styling and configured PostCSS.
- Updated TypeScript configuration for better compatibility with Next.js.
- Removed unused files and configurations related to the previous setup.

src/lib/relay-utils.ts

@@ -0,0 +1,49 @@
+export interface RelayOptions {
+  stripHeaders?: string[];
+}
+
+export function normalizeTargetUrl(target: string | null, relayPath: string): string | null {
+  if (!target) return null;
+  return target.replace(/\/$/, "") + relayPath;
+}
+
+export function stripRelayHeaders(headers: Headers): Headers {
+  const stripped = new Headers(headers);
+  stripped.delete("x-relay-target");
+  stripped.delete("x-relay-path");
+  stripped.delete("host");
+  return stripped;
+}
+
+export function shouldSendBody(method: string): boolean {
+  return method !== "GET" && method !== "HEAD";
+}
+
+export function buildRelayRequest(
+  req: Request,
+  targetUrl: string,
+  headers: Headers
+): RequestInit {
+  return {
+    method: req.method,
+    headers,
+    body: shouldSendBody(req.method) ? req.body : undefined,
+    duplex: "half" as const,
+  };
+}
+
+export function isAllowedTarget(url: string): boolean {
+  try {
+    const parsed = new URL(url);
+    return ["http:", "https:"].includes(parsed.protocol);
+  } catch {
+    return false;
+  }
+}
+
+export function createRelayResponse(response: Response): Response {
+  return new Response(response.body, {
+    status: response.status,
+    headers: response.headers,
+  });
+}