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.

.cursor/rules/testing.mdc

@@ -2,7 +2,7 @@
 description: Rules for writing and running tests
 globs:
   - tests/**
-  - webapp/js/**/*.test.js
+  - webapp-next/src/**/*.test.{ts,tsx}
 ---
 
 # Testing
@@ -51,48 +51,28 @@ def test_get_or_create_user_creates_new(test_db_url):
         assert user.telegram_user_id == 123
 ```
 
-## JavaScript tests (Vitest)
+## Frontend tests (Vitest + React Testing Library)
 
 ### Configuration
 
-- Config: `webapp/vitest.config.js`.
-- Environment: `happy-dom` (lightweight DOM implementation).
-- Test files: `webapp/js/**/*.test.js`.
-- Run: `npm run test` (from `webapp/`).
+- Config: `webapp-next/vitest.config.ts`.
+- Environment: jsdom; React Testing Library for components.
+- Test files: `webapp-next/src/**/*.test.{ts,tsx}` (co-located or in test files).
+- Setup: `webapp-next/src/test/setup.ts`.
+- Run: `cd webapp-next && npm test` (or `npm run test`).
 
-### DOM setup
+### Writing frontend tests
 
-Modules that import from `dom.js` expect DOM elements to exist at import time.
-Use `beforeAll` to set up the required HTML structure before the first import:
-
-```js
-import { beforeAll, describe, it, expect } from "vitest";
-
-beforeAll(() => {
-  document.body.innerHTML = `
-    <div id="calendar"></div>
-    <h2 id="monthTitle"></h2>
-    <button id="prevBtn"></button>
-    <button id="nextBtn"></button>
-    <div id="dutyList"></div>
-    <div id="dayDetail"></div>
-    <div id="accessDenied" class="hidden"></div>
-    <div id="errorBanner" class="hidden"></div>
-  `;
-});
-```
-
-### Writing JS tests
-
-- File naming: `webapp/js/<module>.test.js` (co-located with the source module).
-- Test pure functions first (`dateUtils`, `i18n`, `utils`); mock DOM for render tests.
-- Use `describe` blocks to group by function, `it` blocks for individual cases.
+- Pure lib modules: unit test with Vitest (`describe` / `it` / `expect`).
+- React components: use `@testing-library/react` (render, screen, userEvent); wrap with required providers (e.g. TelegramProvider, store) via `src/test/test-utils.tsx` where needed.
+- Mock Telegram SDK and API fetch where necessary.
+- File naming: `<module>.test.ts` or `<Component>.test.tsx`.
 
 ### Example structure
 
-```js
+```ts
 import { describe, it, expect } from "vitest";
-import { localDateString } from "./dateUtils.js";
+import { localDateString } from "./date-utils";
 
 describe("localDateString", () => {
   it("formats date as YYYY-MM-DD", () => {