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/components/states/AccessDenied.tsx

@@ -0,0 +1,46 @@
+/**
+ * Access denied state: message and optional server detail.
+ * Ported from webapp/js/ui.js showAccessDenied and states.css .access-denied.
+ */
+
+"use client";
+
+import { useTranslation } from "@/i18n/use-translation";
+import { cn } from "@/lib/utils";
+
+export interface AccessDeniedProps {
+  /** Optional detail from API 403 response, shown below the main message. */
+  serverDetail?: string | null;
+  /** Optional class for the container. */
+  className?: string;
+}
+
+/**
+ * Displays access denied message; optional second paragraph for server detail.
+ */
+export function AccessDenied({ serverDetail, className }: AccessDeniedProps) {
+  const { t } = useTranslation();
+  const hasDetail = Boolean(serverDetail && String(serverDetail).trim());
+
+  return (
+    <div
+      className={cn(
+        "rounded-xl bg-surface py-6 px-4 my-3 text-center text-muted-foreground shadow-sm transition-opacity duration-200",
+        className
+      )}
+      role="alert"
+    >
+      <p className="m-0 mb-2 font-semibold text-error">
+        {t("access_denied")}
+      </p>
+      {hasDetail && (
+        <p className="mt-2 m-0 text-sm text-muted">
+          {serverDetail}
+        </p>
+      )}
+      <p className="mt-2 m-0 text-sm text-muted">
+        {t("access_denied.hint")}
+      </p>
+    </div>
+  );
+}