feat: scaffold monorepo with Go backend + Next.js 15 frontend

- Root monorepo: /backend (Go) + /frontend (Next.js 15)
- Go module: mgit.msbls.de/m/KanzlAI with minimal HTTP server
- Next.js 15: TypeScript strict, Tailwind CSS v4, App Router, Bun
- Root Makefile: dev, build, lint, test targets
- Root .gitignore covering Go, Node, IDE, OS files
- CLAUDE.md updated with project structure and tech stack
- .claude/CLAUDE.md with coding conventions (Go stdlib style, TS strict)

backend/cmd/server/main.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+)
+
+func main() {
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = "8080"
+	}
+
+	http.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprintf(w, "ok")
+	})
+
+	log.Printf("Starting KanzlAI API server on :%s", port)
+	if err := http.ListenAndServe(":"+port, nil); err != nil {
+		log.Fatal(err)
+	}
+}