feat: migrate to Next.js for Mini App and enhance project structure

- Replaced the previous webapp with a new Mini App built using Next.js, improving performance and maintainability.
- Updated the `.gitignore` to exclude Next.js build artifacts and node modules.
- Revised documentation in `AGENTS.md`, `README.md`, and `architecture.md` to reflect the new Mini App structure and technology stack.
- Enhanced Dockerfile to support the new build process for the Next.js application.
- Updated CI workflow to build and test the Next.js application.
- Added new configuration options for the Mini App, including `MINI_APP_SHORT_NAME` for improved deep linking.
- Refactored frontend testing setup to accommodate the new structure and testing framework.
- Removed legacy webapp files and dependencies to streamline the project.

webapp-next/src/app/not-found.tsx

@@ -0,0 +1,25 @@
+/**
+ * Next.js 404 page. Shown when notFound() is called or route is unknown.
+ * For static export with a single route this is rarely hit; added for consistency.
+ */
+
+"use client";
+
+import Link from "next/link";
+import { useTranslation } from "@/i18n/use-translation";
+
+export default function NotFound() {
+  const { t } = useTranslation();
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center gap-4 bg-background px-4 text-foreground">
+      <h1 className="text-xl font-semibold">{t("not_found.title")}</h1>
+      <p className="text-muted-foreground">{t("not_found.description")}</p>
+      <Link
+        href="/"
+        className="rounded-lg bg-primary px-4 py-2 text-primary-foreground hover:bg-primary/90"
+      >
+        {t("not_found.open_calendar")}
+      </Link>
+    </div>
+  );
+}