feat: Initial implementation of Annas Rechnungsmanager

Full-stack German accounting & invoice management web application:

- Multi-company management (Mandantenverwaltung) with full CRUD
- Invoice creation with dynamic line items and automatic tax calculation
- Sequential invoice numbering per company (RE-2024-001 format)
- §14 UStG compliant PDF invoice generation via @react-pdf/renderer
- Customer management (Kundenverwaltung) per company
- Tax reports: quarterly USt-Voranmeldung and monthly revenue overview
- Email/password authentication via NextAuth.js v5
- Responsive, modern UI with Tailwind CSS and custom shadcn/ui components
- Prisma v5 ORM with MySQL/MariaDB schema + demo seed data

Stack: Next.js 14 (App Router) · TypeScript · Prisma/MySQL · NextAuth.js

https://claude.ai/code/session_01FN53KKxo5ebrGwqFhxzkT9

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