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>

next.config.ts

@@ -0,0 +1,25 @@
+import type { NextConfig } from "next";
+import { networkInterfaces } from "os";
+
+// Next.js 16's dev server blocks cross-origin access to dev-only resources
+// (the HMR websocket, etc.) unless the origin is allowlisted - which silently
+// breaks hydration when testing from a real phone over WiFi, since the phone's
+// origin is the dev machine's LAN IP, never "localhost". Derived dynamically
+// because DHCP can hand out a different IP on a later session. Only affects
+// `next dev`. Copied from nachklang-feedback.
+const lanIps = Object.values(networkInterfaces())
+  .flat()
+  .filter((iface): iface is NonNullable<typeof iface> => !!iface && iface.family === "IPv4" && !iface.internal)
+  .map((iface) => iface.address);
+
+const nextConfig: NextConfig = {
+  // Self-contained server.js for Plesk's Passenger Node.js toolkit, the same
+  // way nachklang-feedback is deployed.
+  output: "standalone",
+  allowedDevOrigins: lanIps,
+  turbopack: {
+    root: __dirname,
+  },
+};
+
+export default nextConfig;