[MEL]: Make preview source selector optional

[cmoinier]

Description

This PR makes the source selector for the preview (map & data) optional.
This does not change anything in generic gn-ui visually, but will allow the MEL instance to hide the selector once the latest version is upgraded there. If the external viewer is not present, the space automatically collapses.
Note that this could have been done directly in MEL but this feature could be used by other instances on gn-ui in the future.

Architectural changes

none

Screenshots

no UI changes in gn-ui

Quality Assurance Checklist

• Commit history is devoid of any merge commits and readable to facilitate reviews
• If new logic ⚙️ is introduced: unit tests were added
• If new user stories 🤏 are introduced: E2E tests were added
• If new UI components 🕹️ are introduced: corresponding stories in Storybook were created
• If breaking changes 🪚 are introduced: add the breaking change label
• If bugs 🐞 are fixed: add the backport <release branch> label
• The documentation website 📚 has received the love it deserves

This work is sponsored by MEL.

[github-actions]

Affected libs: feature-record,
Affected apps: metadata-editor,

• 🚀 Build and deploy storybook and demo on GitHub Pages
• 📦 Build and push affected docker images

[github-actions]

📷 Screenshots are here!

[tkohr]

Thanks @cmoinier !

[coveralls]

coverage: 84.216%. first build
when pulling 063c1e4 on mel-source-selector-optional
into f9b0474 on main.