feat(ui): remember whether to start with shown annotations on Dashboard page

[sranka]

Closes #5966

Briefly describe your proposed changes: This PR enhances the UI so that it remembers

• if the annotations control bar is opened on a Dashboard page
• whether graph annotations are shown or hidden

and restores the Dashboard page with the remembered state, even on page reload.

• [x] CHANGELOG.md updated with a link to the PR (not the Issue)
• [x] Well-formatted commit messages
• [x] Rebased/mergeable
• [x] Tests pass