Calculate reference dem with QGIS

[Joonalai]

Description

This PR still lacks some things:

• e2e checking that reference layers are available to add
• There is some issue with blast2dem in tile corners. It does not consider neighbors correctly:

Developer Checklist

• Each commit introduces changes only to a single monorepo component and is scoped using conventional commits (e.g. feat(db): ...) when possible
• A screencast is included when the PR contains UI changes to the QGIS plugin