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/lib/calendar-data.ts

@@ -0,0 +1,50 @@
+/**
+ * Pure functions: group duties and calendar events by local date (YYYY-MM-DD).
+ * Ported from webapp/js/calendar.js (dutiesByDate, calendarEventsByDate).
+ */
+
+import type { CalendarEvent, DutyWithUser } from "@/types";
+import { localDateString } from "./date-utils";
+
+/** Max days to iterate per duty; prevents infinite loop on corrupted API data (end_at < start_at). */
+const MAX_DAYS_PER_DUTY = 366;
+
+/**
+ * Build { localDateKey -> array of summary strings }. API date is UTC YYYY-MM-DD; map to local.
+ */
+export function calendarEventsByDate(
+  events: CalendarEvent[] | null | undefined
+): Record<string, string[]> {
+  const byDate: Record<string, string[]> = {};
+  (events ?? []).forEach((e) => {
+    const utcMidnight = new Date(e.date + "T00:00:00Z");
+    const key = localDateString(utcMidnight);
+    if (!byDate[key]) byDate[key] = [];
+    if (e.summary) byDate[key].push(e.summary);
+  });
+  return byDate;
+}
+
+/**
+ * Group duties by local date (start_at/end_at are UTC).
+ */
+export function dutiesByDate(duties: DutyWithUser[]): Record<string, DutyWithUser[]> {
+  const byDate: Record<string, DutyWithUser[]> = {};
+  duties.forEach((d) => {
+    const start = new Date(d.start_at);
+    const end = new Date(d.end_at);
+    if (end < start) return;
+    const endLocal = localDateString(end);
+    let cursor = new Date(start);
+    let iterations = 0;
+    while (iterations <= MAX_DAYS_PER_DUTY) {
+      const key = localDateString(cursor);
+      if (!byDate[key]) byDate[key] = [];
+      byDate[key].push(d);
+      if (key === endLocal) break;
+      cursor.setDate(cursor.getDate() + 1);
+      iterations++;
+    }
+  });
+  return byDate;
+}