Various diagnostic UI improvements

[erri120]

Part of #1091.

• Fixes the horizontal line coloring:

• Adds chips with counts and filters
• Adds text for no diagnostics
• Adds the summary as a header to the details page:
• Adds some basic Markdown styling:

[Al12rs]

Could you add a "No Entries" text if there are no diagnostics to see in a given filtered view?
Use a Subdued foreground color and pick a text style

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 84 lines in your changes are missing coverage. Please review.

Project coverage is 53.18%. Comparing base (ab1c879) to head (f95d363).
Report is 7 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1115      +/-   ##
==========================================
- Coverage   53.97%   53.18%   -0.80%     
==========================================
  Files         673      674       +1     
  Lines       22349    22431      +82     
  Branches     1716     1723       +7     
==========================================
- Hits        12063    11929     -134     
- Misses       9894    10102     +208     
- Partials      392      400       +8     

Flag	Coverage Δ
Linux	53.10% <0.00%> (-0.21%)	⬇️
Windows	?
clean_environment_tests	53.17% <0.00%> (-0.79%)	⬇️
macOS	52.52% <0.00%> (-0.25%)	⬇️
network_tests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...I/Controls/Diagnostics/DiagnosticEntryViewModel.cs	0.00% <0.00%> (ø)
.../Diagnostics/Details/DiagnosticDetailsViewModel.cs	0.00% <0.00%> (ø)
...c/NexusMods.App.UI/Extensions/ClassesExtensions.cs	0.00% <0.00%> (ø)
...rc/NexusMods.App.UI/Resources/Language.Designer.cs	48.56% <0.00%> (-2.19%)	⬇️
.../Pages/Diagnostics/List/DiagnosticListViewModel.cs	0.00% <0.00%> (ø)

... and 31 files with indirect coverage changes

[Al12rs]

Did you figure out the chip behavior with Design?

[erri120]

Did you figure out the chip behavior with Design?

Yeah, they now work similar to tabs, meaning you can only have one filter active at a time.