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)

frontend/src/app/page.tsx

@@ -0,0 +1,7 @@
+export default function Home() {
+  return (
+    <main className="flex min-h-screen items-center justify-center">
+      <h1 className="text-4xl font-bold">KanzlAI</h1>
+    </main>
+  );
+}