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)

.gitignore

@@ -0,0 +1,47 @@
+# Go
+backend/bin/
+*.exe
+*.dll
+*.so
+*.dylib
+
+# Node / Next.js
+frontend/node_modules/
+frontend/.next/
+frontend/out/
+frontend/build/
+frontend/.env*
+
+# Dependencies
+node_modules/
+
+# Testing
+coverage/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Debug logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+
+# Env
+.env
+.env.*
+!.env.example
+
+# Build artifacts
+dist/
+tmp/
+
+# TypeScript
+*.tsbuildinfo