Improve Linux setup and modernize project tooling

[AdolfMacro]

Summary

• Add run.sh for Linux setup and execution:
  • --install installs/checks npm dependencies without building or starting the app.
  • --local builds and runs Chainer locally.
  • --docker starts Chainer with MongoDB.
  • Automatically selects a free port, with an option to specify a preferred port.
  • Generates private application secrets in .env.local when missing.
• Move provider-fixture recording from JavaScript to Python. Python is only needed for fixture recording and its offline tests—not to run Chainer.
• Migrate request-ID handling to the Next.js 16 proxy.ts convention.
• Upgrade Nodemailer and patch vulnerable dependencies.
• Extend CI with Python tests and a Windows desktop build job.
• Update English and German documentation.

Validation

• npm run check — passed: typecheck, lint, 359 tests passed, 2 skipped, 5 Python tests, and production build.
• npm audit — 0 vulnerabilities.
• Docker runner tested; it selected the next available port when the preferred port was occupied, and the health endpoint returned ok.