Exclude/flattened not obvious in file node explorer

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# Feature request

Currently the behavior of exclude/flattened files in the node explorer is inconsistent and confusing to novice users. This was a user feedback a while back. Can we make it more obvious and consistent?

## Description

As a novice user, I want consistent and obvious behavior in the node explorer, so that I can understand what is going on.

## Acceptance criteria
- When I flatten something, the node explorer displays an explaining tooltip why it's gray (currently nothing) and displays the same flatten icon that I used to flatten it in the context menu. The icon is not behind the three dots burger menu as it is currently.

Current:

- When I exclude something, the node explorer displays an explaining tooltip why it's gray (currently shows "no area metrics" which is wrong) and displays the same exclude icon that I used to exclude it in the context menu (currently shows not .

- When a file does not have an area metric we can perhaps improve the tooltip to say: "Does not have area metric."
- Right-clicking a file that does not have an area metrics opens a small modal (currently nothing happens, which is confusing), same style as the context menu. This modal says something like: "The file/folder cannot be visualized in the map because it does not have the selected area metric. Either select a different area metric or see if another analyzer can generate the metric for you."

Start in the node explorer and its context-menu handling, tracing the existing flatten, exclude, and missing-area-metric states. Compare the displayed icons, gray-state tooltips, and right-click behavior with the acceptance criteria; done means each state explains itself and uses the requested icon or modal.

Written by the indexing model from the issue text.