Trufos Exporter

**Is your feature request related to a problem? Please describe.**
We currently only support sharing collections via GitHub but no direct exports/imports.

**Describe the solution you'd like**
- Support exporting a collection in the backend
- Add export button to UI
- The export format should be a `.trufos.zip` file that contains the whole collection directory with either
- secrets in plain text
- secrets removed
- OPTIONAL: Think about supporting passwords on the ZIP for security reasons

**Describe alternatives you've considered**
We could provide any other binary or text-based (e.g. JSON) format. But since we already have a robust FS reading system, it makes sense to use it.

**Additional context**
- [zip.js](https://www.npmjs.com/package/@zip.js/zip.js) seems like a fitting library. It also supports ZIP encryption

No files, tests, or entry points are named. Start by locating the backend collection filesystem handling and the UI collection actions, then define the export/import scope, secret-handling behavior, and whether ZIP passwords are included; done means a collection can be exported and imported in the specified format.

Written by the indexing model from the issue text.