The sidebar of the selected folder shows area distribution

# Feature request

Issue #4175 describes how the distribution bar changes on hover to show the file extension distribution per sub-folder. But having it only on hover can be a bit annoying. It would be nice to click a folder and see the distribution there permanently. Perhaps even take a screenshot for a presentation. Perhaps hover over another folder to compare its distribution to the selected folder. For a presentation it would also be great if the distribution is not a bar but a nice pie chart.

## Description

As an auditor, I want to see a pie chart of the file extension distribution of a folder.

It could be between the folder name and the primary metrics as seen here:

## Acceptance criteria

1a. WHEN I select a folder in the map, the pie chart should show the file extension distribution of the selected area metric
1b. WHEN I select a folder in the file/node explorer, the pie chart should show the file extension distribution of the selected area metric
2. WHEN I select a file, the pie chart should not be shown.
3. WHEN I select a folder, the folder name should be clearly visible in the side bar. This is already done for files but not currently for folders.

## Examples
(1)
GIVEN I have selected the area metric "RLoC"
WHEN I select a folder with 1000 RloC of which 900 RLoC are .cs and 100 RLoC are .html files
THEN the pie chart should change to 90% cs and 10% html

## Assumptions & Exclusions

## Development notes (optional Task Breakdown)
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## Open questions

Start by locating the sidebar, folder-selection handling in the map and file/node explorer, and the existing hover-based file-extension distribution. Implement the selected-folder view and verify it against the acceptance criteria: pie chart for the selected area metric, no chart for files, and a visible folder name.

Written by the indexing model from the issue text.