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>

README.md

@@ -1,27 +1,31 @@
-# NachklangCalendar
+# Nachklang Calendar
 
-This project was generated with [Angular CLI](https://github.com/angular/angular-cli) version 14.2.1.
+Event calendar for Nachklang e.V.: a public landing page for subscribing to the calendar, and an
+admin area (`/admin`) for editing events in all calendars.
 
-## Development server
+Next.js (App Router) + TypeScript + Tailwind + shadcn, the same stack as `nachklang-feedback`.
+Rewritten from Angular in September 2026 ([docs/rewrite-plan.md](docs/rewrite-plan.md)).
 
-Run `ng serve` for a dev server. Navigate to `http://localhost:4200/`. The application will automatically reload if you change any of the source files.
+Documentation: [architecture](docs/architecture.md) · [features](docs/features.md) ·
+[gotchas](docs/gotchas.md) · [deployment](docs/deployment.md) · [API wish list](docs/api-wishlist.md)
 
-## Code scaffolding
+## Development
 
-Run `ng generate component component-name` to generate a new component. You can also use `ng generate directive|pipe|service|class|guard|interface|enum|module`.
+```bash
+cp .env.local.example .env.local   # then adjust if needed
+npm install
+npm run dev                        # http://localhost:4200
+```
 
-## Build
+Port 4200 is not arbitrary: it is the port the API's better-auth origins and the admin app's
+redirect allow-list trust. Another port breaks sign-out and the return redirect after sign-in.
 
-Run `ng build` to build the project. The build artifacts will be stored in the `dist/` directory.
+The admin area needs the API (`:3000`) and the admin app (`:3002`) running locally.
 
-## Running unit tests
+## Other scripts
 
-Run `ng test` to execute the unit tests via [Karma](https://karma-runner.github.io).
-
-## Running end-to-end tests
-
-Run `ng e2e` to execute the end-to-end tests via a platform of your choice. To use this command, you need to first add a package that implements end-to-end testing capabilities.
-
-## Further help
-
-To get more help on the Angular CLI use `ng help` or go check out the [Angular CLI Overview and Command Reference](https://angular.io/cli) page.
+```bash
+npm run build   # production build (output: standalone; see docs/deployment.md)
+npm run lint
+npm test        # Vitest, pure logic in lib/
+```