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.tsx

@@ -0,0 +1,93 @@
+/**
+ * 6-week (42-cell) calendar grid starting from Monday. Composes CalendarDay cells.
+ * Ported from webapp/js/calendar.js renderCalendar.
+ */
+
+"use client";
+
+import { useMemo } from "react";
+import {
+  firstDayOfMonth,
+  getMonday,
+  localDateString,
+} from "@/lib/date-utils";
+import type { CalendarEvent, DutyWithUser } from "@/types";
+import { dutiesByDate, calendarEventsByDate } from "@/lib/calendar-data";
+import { cn } from "@/lib/utils";
+import { CalendarDay } from "./CalendarDay";
+
+export interface CalendarGridProps {
+  /** Currently displayed month. */
+  currentMonth: Date;
+  /** All duties for the visible range (will be grouped by date). */
+  duties: DutyWithUser[];
+  /** All calendar events for the visible range. */
+  calendarEvents: CalendarEvent[];
+  /** Called when a day cell is clicked (opens day detail). Receives date key and cell rect for popover. */
+  onDayClick: (dateKey: string, anchorRect: DOMRect) => void;
+  className?: string;
+}
+
+const CELLS = 42;
+
+export function CalendarGrid({
+  currentMonth,
+  duties,
+  calendarEvents,
+  onDayClick,
+  className,
+}: CalendarGridProps) {
+  const dutiesByDateMap = useMemo(
+    () => dutiesByDate(duties),
+    [duties]
+  );
+  const calendarEventsByDateMap = useMemo(
+    () => calendarEventsByDate(calendarEvents),
+    [calendarEvents]
+  );
+  const todayKey = localDateString(new Date());
+
+  const cells = useMemo(() => {
+    const first = firstDayOfMonth(currentMonth);
+    const start = getMonday(first);
+    const result: { date: Date; key: string; month: number }[] = [];
+    const d = new Date(start);
+    for (let i = 0; i < CELLS; i++) {
+      const key = localDateString(d);
+      result.push({ date: new Date(d), key, month: d.getMonth() });
+      d.setDate(d.getDate() + 1);
+    }
+    return result;
+  }, [currentMonth]);
+
+  return (
+    <div
+      className={cn(
+        "calendar-grid grid grid-cols-7 gap-1 mb-4 min-h-[var(--calendar-grid-min-height)]",
+        className
+      )}
+      role="grid"
+      aria-label="Calendar"
+    >
+      {cells.map(({ date, key, month }) => {
+        const isOtherMonth = month !== currentMonth.getMonth();
+        const dayDuties = dutiesByDateMap[key] ?? [];
+        const eventSummaries = calendarEventsByDateMap[key] ?? [];
+
+        return (
+          <div key={key} role="gridcell" className="min-h-0">
+            <CalendarDay
+              dateKey={key}
+              dayOfMonth={date.getDate()}
+              isToday={key === todayKey}
+              isOtherMonth={isOtherMonth}
+              duties={dayDuties}
+              eventSummaries={eventSummaries}
+              onDayClick={onDayClick}
+            />
+          </div>
+        );
+      })}
+    </div>
+  );
+}