[Spike] - Improve behavior of "Add Data" button in the header

Overview of the Feature Request
In the weekly meeting on Aug 28, 2025, we discussed the Add Data button in the header, which @sekmiller raised. The issue is that the button always directs users to add a collection or dataset in ROOT, rather than in the collection they are currently viewing. This has caused confusion, since many users don’t expect new data to be placed in the root collection.

Given our goal of encouraging users to create their own collections first, we may need to introduce an additional step that guides them to create a collection before adding datasets. This would make the flow more intuitive and friendly for new users.

What kind of user is the feature intended for?
UI users

What inspired the request?
weekly meeting discussion topic

What existing behavior do you want changed?
the "Add Data" button in header

No file, test, or entry point is named. Start by reproducing the header's Add Data flow while viewing a non-root collection, then document whether the flow should use the current collection or guide users through collection creation. Done means the intended behavior and implementation scope are agreed.