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.

api/index.ts

@@ -1,50 +0,0 @@
-import {
-	buildRelayRequest,
-	createRelayResponse,
-	normalizeTargetUrl,
-	stripRelayHeaders,
-	isAllowedTarget,
-} from "../relay-utils";
-
-export const config = { runtime: "edge" };
-
-const ALLOWED_METHODS = new Set(["GET", "POST", "PUT", "DELETE", "PATCH", "HEAD", "OPTIONS"]);
-
-export default async function handler(req: Request): Promise<Response> {
-	if (!ALLOWED_METHODS.has(req.method)) {
-		return new Response(JSON.stringify({ error: "Method not allowed" }), {
-			status: 405,
-			headers: { "content-type": "application/json" },
-		});
-	}
-
-	const target = req.headers.get("x-relay-target");
-	const relayPath = req.headers.get("x-relay-path") || "/";
-
-	const targetUrl = normalizeTargetUrl(target, relayPath);
-	if (!targetUrl) {
-		return new Response(
-			JSON.stringify({ error: "Missing x-relay-target header" }),
-			{
-				status: 400,
-				headers: { "content-type": "application/json" },
-			},
-		);
-	}
-
-	if (!isAllowedTarget(targetUrl)) {
-		return new Response(
-			JSON.stringify({ error: "Target domain not allowed" }),
-			{
-				status: 403,
-				headers: { "content-type": "application/json" },
-			},
-		);
-	}
-
-	const headers = stripRelayHeaders(new Headers(req.headers));
-	const fetchOptions = buildRelayRequest(req, targetUrl, headers);
-
-	const response = await fetch(targetUrl, fetchOptions);
-	return createRelayResponse(response);
-}