Add 'Open saved session' button to start screen on desktop

[cmdcolin]

fixes #3185

Also uses a MUI grid to vertically center the button, checkbox, etc. (which is a technique I hadn't known before, could apply to other places in app potentially where that is relevant)

[codecov]

Codecov Report

Merging #3188 (3c29d5c) into main (8d4fa57) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

❗ Current head 3c29d5c differs from pull request most recent head 0120028. Consider uploading reports for the commit 0120028 to get more accurate results

@@            Coverage Diff             @@
##             main    #3188      +/-   ##
==========================================
- Coverage   59.50%   59.48%   -0.02%     
==========================================
  Files         671      671              
  Lines       28729    28738       +9     
  Branches     6966     6968       +2     
==========================================
  Hits        17094    17094              
- Misses      11313    11322       +9     
  Partials      322      322              

Impacted Files	Coverage Δ
...se-desktop/src/StartScreen/RecentSessionsPanel.tsx	0.00% <0.00%> (ø)
products/jbrowse-web/src/util.ts	48.33% <0.00%> (ø)

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