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/calendar/CalendarGrid.test.tsx

@@ -0,0 +1,88 @@
+/**
+ * Unit tests for CalendarGrid: 42 cells, data-date, today class, month title in header.
+ * Ported from webapp/js/calendar.test.js renderCalendar.
+ */
+
+import { describe, it, expect, beforeEach } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { CalendarGrid } from "./CalendarGrid";
+import { CalendarHeader } from "./CalendarHeader";
+import { resetAppStore } from "@/test/test-utils";
+
+describe("CalendarGrid", () => {
+  beforeEach(() => {
+    resetAppStore();
+  });
+
+  it("renders 42 cells (6 weeks)", () => {
+    const currentMonth = new Date(2025, 0, 1); // January 2025
+    render(
+      <CalendarGrid
+        currentMonth={currentMonth}
+        duties={[]}
+        calendarEvents={[]}
+        onDayClick={() => {}}
+      />
+    );
+    const cells = screen.getAllByRole("button", { name: /;/ });
+    expect(cells.length).toBe(42);
+  });
+
+  it("sets data-date on each cell to YYYY-MM-DD", () => {
+    const currentMonth = new Date(2025, 0, 1);
+    render(
+      <CalendarGrid
+        currentMonth={currentMonth}
+        duties={[]}
+        calendarEvents={[]}
+        onDayClick={() => {}}
+      />
+    );
+    const grid = screen.getByRole("grid", { name: "Calendar" });
+    const buttons = grid.querySelectorAll('button[data-date]');
+    expect(buttons.length).toBe(42);
+    buttons.forEach((el) => {
+      const date = el.getAttribute("data-date");
+      expect(date).toMatch(/^\d{4}-\d{2}-\d{2}$/);
+    });
+  });
+
+  it("adds today styling to cell matching today", () => {
+    const today = new Date();
+    const currentMonth = new Date(today.getFullYear(), today.getMonth(), 1);
+    render(
+      <CalendarGrid
+        currentMonth={currentMonth}
+        duties={[]}
+        calendarEvents={[]}
+        onDayClick={() => {}}
+      />
+    );
+    const year = today.getFullYear();
+    const month = String(today.getMonth() + 1).padStart(2, "0");
+    const day = String(today.getDate()).padStart(2, "0");
+    const todayKey = `${year}-${month}-${day}`;
+    const todayCell = document.querySelector(`button[data-date="${todayKey}"]`);
+    expect(todayCell).toBeTruthy();
+    expect(todayCell?.className).toMatch(/today|bg-today/);
+  });
+});
+
+describe("CalendarHeader", () => {
+  beforeEach(() => {
+    resetAppStore();
+  });
+
+  it("sets month title from lang and given year/month", () => {
+    render(
+      <CalendarHeader
+        month={new Date(2025, 1, 1)}
+        onPrevMonth={() => {}}
+        onNextMonth={() => {}}
+      />
+    );
+    const heading = screen.getByRole("heading", { level: 1 });
+    expect(heading).toHaveTextContent("February");
+    expect(heading).toHaveTextContent("2025");
+  });
+});