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)

.claude/CLAUDE.md

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+# KanzlAI Coding Conventions
+
+## Go (backend/)
+
+- Standard library style — prefer stdlib over third-party packages where reasonable
+- `go vet` and `go test` must pass before committing
+- Package layout: `cmd/` for entry points, `internal/` for private packages
+- Error handling: return errors, don't panic. Wrap with context via `fmt.Errorf("operation: %w", err)`
+- Naming: follow Go conventions (MixedCaps, not snake_case)
+- No global mutable state
+
+## TypeScript / Next.js (frontend/)
+
+- TypeScript strict mode (enabled in tsconfig.json)
+- App Router (src/app/) — no Pages Router
+- Use server components by default, `"use client"` only when needed
+- Tailwind CSS v4 for styling — no CSS modules or styled-components
+- ESLint must pass before committing
+- Import aliases: `@/` maps to `src/`
+- Bun as package manager (not npm/yarn/pnpm)
+
+## General
+
+- No secrets in code — use environment variables
+- Commit messages: `feat:`, `fix:`, `build:`, `refactor:`, `docs:`, `test:`, `chore:`