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/ErrorState.tsx

@@ -0,0 +1,75 @@
+/**
+ * Error state: warning icon, message, and optional Retry button.
+ * Ported from webapp/js/ui.js showError and states.css .error.
+ */
+
+"use client";
+
+import { useTranslation } from "@/i18n/use-translation";
+import { Button } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
+
+export interface ErrorStateProps {
+  /** Error message to display. If not provided, uses generic i18n message. */
+  message?: string | null;
+  /** Optional retry callback; when provided, a Retry button is shown. */
+  onRetry?: (() => void) | null;
+  /** Optional class for the container. */
+  className?: string;
+}
+
+/** Warning triangle icon 24×24 for error state. */
+function ErrorIcon({ className }: { className?: string }) {
+  return (
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      width="24"
+      height="24"
+      viewBox="0 0 24 24"
+      fill="none"
+      stroke="currentColor"
+      strokeWidth="2"
+      strokeLinecap="round"
+      strokeLinejoin="round"
+      className={cn("shrink-0 text-error", className)}
+      aria-hidden
+    >
+      <path d="M10.29 3.86L1.82 18a2 2 0 0 0 1.71 3h16.94a2 2 0 0 0 1.71-3L13.71 3.86a2 2 0 0 0-3.42 0z" />
+      <line x1="12" y1="9" x2="12" y2="13" />
+      <line x1="12" y1="17" x2="12.01" y2="17" />
+    </svg>
+  );
+}
+
+/**
+ * Displays an error message with optional Retry button.
+ */
+export function ErrorState({ message, onRetry, className }: ErrorStateProps) {
+  const { t } = useTranslation();
+  const displayMessage =
+    message && String(message).trim() ? message : t("error_generic");
+
+  return (
+    <div
+      className={cn(
+        "flex flex-col items-center gap-3 rounded-xl bg-surface py-5 px-4 my-3 text-center text-error transition-opacity duration-200",
+        className
+      )}
+      role="alert"
+    >
+      <ErrorIcon />
+      <p className="m-0 text-sm font-medium">{displayMessage}</p>
+      {typeof onRetry === "function" && (
+        <Button
+          type="button"
+          variant="default"
+          size="sm"
+          className="mt-1 bg-primary text-primary-foreground hover:opacity-90 focus-visible:outline-2 focus-visible:outline-accent focus-visible:outline-offset-2"
+          onClick={onRetry}
+        >
+          {t("error.retry")}
+        </Button>
+      )}
+    </div>
+  );
+}