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/LoadingState.test.tsx

@@ -0,0 +1,17 @@
+/**
+ * Unit tests for LoadingState. Ported from webapp/js/ui.test.js (loading).
+ */
+
+import { describe, it, expect, beforeEach } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { LoadingState } from "./LoadingState";
+import { resetAppStore } from "@/test/test-utils";
+
+describe("LoadingState", () => {
+  beforeEach(() => resetAppStore());
+
+  it("renders loading text", () => {
+    render(<LoadingState />);
+    expect(screen.getByText(/Loading/i)).toBeInTheDocument();
+  });
+});