feat: add container image support

[rarguello]

What and why

Add a multi-stage Node.js 24 container image so torlink can run without installing its Node.js dependencies directly on the host.

The image runs as an unprivileged user and provides persistent volumes for application state and downloads. The README documents Docker and Podman usage.

Checklist

• npm run typecheck is clean
• npm test passes
• New logic has a test — not applicable; validated by building the image
• If I added a key, I updated both HELP_GROUPS and footerHints — not applicable
• If I added a Store field, I updated makeStore — not applicable
• OS-touching code works on Windows, macOS, and Linux — not applicable; no host OS integration changed
• One concern, with a Conventional Commits title

Validation

• podman build -t torlink:pr-test .
• npm run typecheck in a temporary container
• npm test in a temporary container: 280 tests passed