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/useListState.ts

@@ -0,0 +1,31 @@
+"use client";
+
+import { usePathname, useRouter, useSearchParams } from "next/navigation";
+import { useCallback, useMemo } from "react";
+import { parseListState, toQueryString, type ListState } from "@/lib/url-state";
+
+/**
+ * The list's filters, read from and written to the query string. `replace`
+ * rather than `push`: flipping a filter should not fill the back button's
+ * history with one entry per click.
+ *
+ * Needs a <Suspense> boundary above it - useSearchParams() in a prerendered
+ * page opts the subtree into client-side rendering.
+ */
+export function useListState(): [ListState, (patch: Partial<ListState>) => void] {
+  const searchParams = useSearchParams();
+  const router = useRouter();
+  const pathname = usePathname();
+
+  const state = useMemo(() => parseListState(new URLSearchParams(searchParams.toString())), [searchParams]);
+
+  const update = useCallback(
+    (patch: Partial<ListState>) => {
+      const query = toQueryString({ ...state, ...patch });
+      router.replace(query ? `${pathname}?${query}` : pathname, { scroll: false });
+    },
+    [state, router, pathname],
+  );
+
+  return [state, update];
+}