Rewrite the calendar frontend in Next.js

Replaces the Angular 18 app with Next.js 16 / React 19 / Tailwind 4 / shadcn,
the same stack and design system as nachklang-feedback, planned in
docs/rewrite-plan.md.

- Admin: all calendars in one mobile-friendly list (month groups, calendar
  chips, Kommend/Vergangen/Alle, search, sort, filters in the URL), an edit
  sheet for create/edit/duplicate/delete, publish confirmation, and an
  "Abonnieren" dialog with subscription links for every calendar.
- Public landing page: subscribe links from env instead of hardcoded, plus
  the next public events, server-rendered and revalidated every 5 minutes.
- All times handled explicitly as Europe/Berlin; whole-day events stored as
  00:00-23:59. Fixes carried over from the Angular app: the move dialog
  leaking a cancelled calendar change, saves that looked saved when they
  failed, in-place Date mutation, and duplicated calendar-id mappings.
- Same shared-session auth as before (proxy.ts gate, /admin/me, 401 vs 403).
- 112 Vitest tests over lib/; docs in docs/ (architecture, features,
  gotchas, API wish list, deployment).

Needs the API branch feature/calendar-ical-escaping-subscriptions deployed
first (GET /calendar/events/subscriptions).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/admin/useEvents.ts

@@ -0,0 +1,82 @@
+"use client";
+
+import { useCallback, useEffect, useRef, useState } from "react";
+import { goToAdminLogin } from "@/lib/admin-auth";
+import { api } from "@/lib/api";
+import { describeApiError, isSessionExpired } from "@/lib/api-errors";
+import { CALENDARS, calendarById } from "@/lib/calendars";
+import type { CalendarEvent } from "@/lib/types";
+
+export type CalendarLoad = { status: "loading" } | { status: "ready" } | { status: "error"; message: string };
+
+const initialLoads = (): Record<number, CalendarLoad> =>
+  Object.fromEntries(CALENDARS.map((c) => [c.id, { status: "loading" } as CalendarLoad]));
+
+/**
+ * Every calendar's events, loaded in parallel - there is no endpoint that
+ * returns them all at once (docs/api-wishlist.md).
+ *
+ * Each calendar loads and fails on its own: one calendar the API cannot serve
+ * shows an error on its chip instead of blanking the whole list. The admin feed
+ * returns exactly the calendar's own events (it ignores `includes_calendars`),
+ * so a calendar's slice of the list can be replaced by calendarId.
+ *
+ * A 401 on any of them means the session is gone, and goes to the login page.
+ */
+export function useEvents() {
+  const [events, setEvents] = useState<CalendarEvent[]>([]);
+  const [loads, setLoads] = useState<Record<number, CalendarLoad>>(initialLoads);
+  // Per-calendar request counter, so a slow response cannot overwrite a newer one.
+  const generation = useRef<Record<number, number>>({});
+
+  const fetchCalendar = useCallback(async (calendarId: number) => {
+    const calendar = calendarById(calendarId);
+    if (!calendar) return;
+    const mine = (generation.current[calendarId] = (generation.current[calendarId] ?? 0) + 1);
+    try {
+      const loaded = await api.getEvents(calendar.slug);
+      if (generation.current[calendarId] !== mine) return;
+      setEvents((current) => [...current.filter((e) => e.calendarId !== calendarId), ...loaded]);
+      setLoads((current) => ({ ...current, [calendarId]: { status: "ready" } }));
+    } catch (e) {
+      if (generation.current[calendarId] !== mine) return;
+      if (isSessionExpired(e)) {
+        goToAdminLogin();
+        return;
+      }
+      setLoads((current) => ({ ...current, [calendarId]: { status: "error", message: describeApiError(e) } }));
+    }
+  }, []);
+
+  useEffect(() => {
+    for (const calendar of CALENDARS) {
+      void fetchCalendar(calendar.id);
+    }
+  }, [fetchCalendar]);
+
+  /** Reload one calendar, or all. Shows the loading state; existing events stay visible meanwhile. */
+  const reload = useCallback(
+    (calendarId?: number) => {
+      const ids = calendarId === undefined ? CALENDARS.map((c) => c.id) : [calendarId];
+      setLoads((current) => ({ ...current, ...Object.fromEntries(ids.map((id) => [id, { status: "loading" }])) }));
+      for (const id of ids) {
+        void fetchCalendar(id);
+      }
+    },
+    [fetchCalendar],
+  );
+
+  /** Re-fetch without flagging the calendar as loading - after a write, to pick up server-side fields. */
+  const refresh = useCallback((calendarId: number) => void fetchCalendar(calendarId), [fetchCalendar]);
+
+  /** Put a saved event into the list (new or changed), replacing any copy with the same id. */
+  const upsertEvent = useCallback((event: CalendarEvent) => {
+    setEvents((current) => [...current.filter((e) => e.eventId !== event.eventId), event]);
+  }, []);
+
+  const removeEvent = useCallback((eventId: number) => {
+    setEvents((current) => current.filter((e) => e.eventId !== eventId));
+  }, []);
+
+  return { events, loads, reload, refresh, upsertEvent, removeEvent };
+}