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>

app/admin/login/page.tsx

@@ -0,0 +1,33 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import { goToAdminLogin } from "@/lib/admin-auth";
+
+/**
+ * This app has no login form: signing in happens once, on admin.nachklang.art,
+ * for every Nachklang app. Copied from nachklang-feedback.
+ *
+ * The route exists as a forwarder so proxy.ts has a stable path on this origin
+ * to send signed-out visitors to, rather than building an external URL inside
+ * the proxy.
+ */
+export default function AdminLoginPage() {
+  // Read from the URL directly rather than with useSearchParams(): this page is
+  // prerendered, and useSearchParams() opts the whole route into client-side
+  // rendering unless it sits inside a <Suspense> boundary.
+  const [redirect] = useState(() =>
+    typeof window === "undefined" ? null : new URLSearchParams(window.location.search).get("redirect"),
+  );
+
+  useEffect(() => {
+    // Passed straight through; the admin app validates it against its own
+    // origin allow-list, and a second check here could only drift from that.
+    goToAdminLogin(redirect || "/admin");
+  }, [redirect]);
+
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center px-6 text-center">
+      <p className="text-sm text-muted-foreground">Weiterleitung zur Nachklang-Anmeldung …</p>
+    </div>
+  );
+}