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>

package.json

@@ -1,40 +1,39 @@
 {
   "name": "nachklang-calendar",
-  "version": "1.1.0",
-  "scripts": {
-    "ng": "ng",
-    "start": "ng serve",
-    "build": "ng build",
-    "watch": "ng build --watch --configuration development",
-    "test": "ng test"
-  },
+  "version": "2.0.0",
   "private": true,
+  "scripts": {
+    "dev": "next dev -p 4200",
+    "build": "next build",
+    "postbuild": "node -e \"require('fs').cpSync('.next/static', '.next/standalone/.next/static', {recursive: true})\"",
+    "start": "next start -p 4200",
+    "lint": "eslint",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
   "dependencies": {
-    "@angular/animations": "^18.0.1",
-    "@angular/common": "^18.0.1",
-    "@angular/compiler": "^18.0.1",
-    "@angular/core": "^18.0.1",
-    "@angular/forms": "^18.0.1",
-    "@angular/material": "^18.0.1",
-    "@angular/platform-browser": "^18.0.1",
-    "@angular/platform-browser-dynamic": "^18.0.1",
-    "@angular/router": "^18.0.1",
-    "@material/dialog": "^15.0.0-canary.7f224ddd4.0",
-    "rxjs": "~7.8.1",
-    "tslib": "^2.6.2",
-    "zone.js": "~0.14.6"
+    "@base-ui/react": "^1.7.0",
+    "@date-fns/tz": "^1.5.0",
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
+    "lucide-react": "^1.33.0",
+    "next": "16.2.12",
+    "react": "19.2.4",
+    "react-dom": "19.2.4",
+    "shadcn": "^4.19.0",
+    "sonner": "^2.0.8",
+    "tailwind-merge": "^3.6.0",
+    "tw-animate-css": "^1.4.0"
   },
   "devDependencies": {
-    "@angular-devkit/build-angular": "^18.0.2",
-    "@angular/cli": "~18.0.2",
-    "@angular/compiler-cli": "^18.0.1",
-    "@types/jasmine": "~5.1.4",
-    "jasmine-core": "~5.1.2",
-    "karma": "~6.4.3",
-    "karma-chrome-launcher": "~3.2.0",
-    "karma-coverage": "~2.2.1",
-    "karma-jasmine": "~5.1.0",
-    "karma-jasmine-html-reporter": "~2.1.0",
-    "typescript": "~5.4.5"
+    "@tailwindcss/postcss": "^4",
+    "@types/node": "^26",
+    "@types/react": "^19",
+    "@types/react-dom": "^19",
+    "eslint": "^9",
+    "eslint-config-next": "16.2.12",
+    "tailwindcss": "^4",
+    "typescript": "^5",
+    "vitest": "^5.0.2"
   }
 }