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.test.tsx

@@ -0,0 +1,26 @@
+/**
+ * Unit tests for ErrorState. Ported from webapp/js/ui.test.js showError.
+ */
+
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { ErrorState } from "./ErrorState";
+import { resetAppStore } from "@/test/test-utils";
+
+describe("ErrorState", () => {
+  beforeEach(() => resetAppStore());
+
+  it("renders error message", () => {
+    render(<ErrorState message="Network error" onRetry={undefined} />);
+    expect(screen.getByText("Network error")).toBeInTheDocument();
+  });
+
+  it("renders Retry button when onRetry provided", () => {
+    const onRetry = vi.fn();
+    render(<ErrorState message="Fail" onRetry={onRetry} />);
+    const retry = screen.getByRole("button", { name: /retry|повторить/i });
+    expect(retry).toBeInTheDocument();
+    retry.click();
+    expect(onRetry).toHaveBeenCalled();
+  });
+});