fix: Sidebar in start custom activity

[Dschoordsch]

Description

Fixes #9440
Make the activity detail view more reactive.

Demo

https://www.loom.com/share/f35fd5fc6a2e44fe8d5450e71d68df76?sid=c6c831d6-2dcc-4524-bb6b-4b4116cb7db5

Testing scenarios

• open a custom activity
• play around with the editing mode and resolutions

Final checklist

• I checked the code review guidelines
• I have added Metrics Representative as reviewer(s) if my PR invovles metrics/data/analytics related changes
• I have performed a self-review of my code, the same way I'd do it for any other team member
• I have tested all cases I listed in the testing scenarios and I haven't found any issues or regressions
• Whenever I took a non-obvious choice I added a comment explaining why I did it this way
• I added the label Skip Maintainer Review Indicating the PR only requires reviewer review and can be merged right after it's approved if the PR introduces only minor changes, does not contain any architectural changes or does not introduce any new patterns and I think one review is sufficient'
• PR title is human readable and could be used in changelog

[nickoferrall]

Tested and it works well

[Dschoordsch]

I saw some issues in the mobile view, so this PR needs another iteration before I can merge it.