Fix centering of the ImportForm on SV inspector

[cmdcolin]

v2.4.2 introduced a regression where the SV inspector import form layout was not centered

This fixes that, and provides a couple of CSS style refactors

current main

this branch

also fixes an issue with "Open split view" not properly showing a disabled state when the features are not TRA/BND events

[cmdcolin]

this one is hard to 'test for' without perhaps doing a puppeteer screenshot, but that could be interesting perhaps

[cmdcolin]

original issue that introduced regression #3610

[codecov]

Codecov Report

Merging #3713 (641a13b) into main (3ef1ed1) will decrease coverage by 0.03%.
The diff coverage is 62.74%.

@@            Coverage Diff             @@
##             main    #3713      +/-   ##
==========================================
- Coverage   64.24%   64.21%   -0.03%     
==========================================
  Files         934      937       +3     
  Lines       29996    29986      -10     
  Branches     7265     7256       -9     
==========================================
- Hits        19271    19257      -14     
- Misses      10560    10564       +4     
  Partials      165      165              

Impacted Files	Coverage Δ
packages/app-core/src/ui/ViewContainer.tsx	83.33% <ø> (ø)
...iew/src/BreakpointSplitView/BreakpointSplitView.ts	72.34% <0.00%> (+2.95%)	⬆️
...src/SvInspectorView/components/SvInspectorView.tsx	66.66% <ø> (-6.07%)	⬇️
...src/SpreadsheetView/components/RowCountMessage.tsx	52.94% <52.94%> (ø)
...-view/src/SpreadsheetView/components/StatusBar.tsx	57.14% <57.14%> (ø)
...view/src/SpreadsheetView/models/SpreadsheetView.ts	65.62% <75.00%> (+0.62%)	⬆️
...ctor/src/SvInspectorView/models/SvInspectorView.ts	70.94% <75.00%> (+0.14%)	⬆️
...SvInspectorView/components/CircularViewOptions.tsx	83.33% <83.33%> (ø)
...src/SpreadsheetView/components/SpreadsheetView.tsx	88.00% <100.00%> (+14.47%)	⬆️

... and 4 files with indirect coverage changes

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