Umstellung auf Next.js (App Router), Design analog werte-next

Führt das getrennte React/Vite-Frontend und Express/MongoDB-Backend in
eine einzelne Next.js-Anwendung zusammen:

- UI: React-Client-Komponenten (TabLayout, AppointmentForm, -List,
  ConfirmationModal) im Look von werte-next (Farben #CCCCFF/#85B7D7/
  #FFFFDD, Rahmenbox, Tabellen-Liste, Footer mit Version/Datum)
- API: Next.js Route Handler unter /api/appointments (GET/POST/PUT/DELETE,
  Soft-Delete beibehalten)
- DB: MongoDB via lib/db.ts (gecachter Client, Standard appointmentsdb)
- Tailwind 4 + TypeScript, Standalone-Docker-Build wie werte-next
- Druckansicht der aktuellen offenen Termine beibehalten
- Deployment: Dockerfile (multi-stage), docker-compose(.prod), deploy.sh

Entfernt die alten frontend/- und backend/-Verzeichnisse.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,47 @@
+# Multi-stage build für die Next.js-Anwendung
+FROM node:22-alpine AS base
+
+# Nur Dependencies installieren, wenn nötig
+FROM base AS deps
+RUN apk add --no-cache libc6-compat
+WORKDIR /app
+
+COPY package.json package-lock.json* ./
+RUN npm ci
+
+# Quellcode bauen
+FROM base AS builder
+WORKDIR /app
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+
+# Build-Datum als Build-Argument
+ARG BUILD_DATE
+ENV NEXT_PUBLIC_BUILD_DATE=${BUILD_DATE}
+
+ENV NEXT_TELEMETRY_DISABLED=1
+
+RUN npm run build
+
+# Produktions-Image
+FROM base AS runner
+WORKDIR /app
+
+ENV NODE_ENV=production
+ENV NEXT_TELEMETRY_DISABLED=1
+
+RUN addgroup --system --gid 1001 nodejs
+RUN adduser --system --uid 1001 nextjs
+
+COPY --from=builder /app/public ./public
+COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+
+USER nextjs
+
+EXPOSE 3000
+
+ENV PORT=3000
+ENV HOSTNAME="0.0.0.0"
+
+CMD ["node", "server.js"]