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)

Makefile

@@ -0,0 +1,45 @@
+.PHONY: dev dev-backend dev-frontend build build-backend build-frontend lint lint-backend lint-frontend test test-backend test-frontend clean
+
+# Development
+dev:
+	@echo "Start backend and frontend separately:"
+	@echo "  make dev-backend"
+	@echo "  make dev-frontend"
+
+dev-backend:
+	cd backend && go run ./cmd/server
+
+dev-frontend:
+	cd frontend && bun run dev
+
+# Build
+build: build-backend build-frontend
+
+build-backend:
+	cd backend && go build -o bin/server ./cmd/server
+
+build-frontend:
+	cd frontend && bun run build
+
+# Lint
+lint: lint-backend lint-frontend
+
+lint-backend:
+	cd backend && go vet ./...
+
+lint-frontend:
+	cd frontend && bun run lint
+
+# Test
+test: test-backend test-frontend
+
+test-backend:
+	cd backend && go test ./...
+
+test-frontend:
+	@echo "No frontend tests configured yet"
+
+# Clean
+clean:
+	rm -rf backend/bin
+	rm -rf frontend/.next frontend/out