Fix matrix layout, default 0px margin

[jmthomas]

closes #668

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.03 ⚠️

Comparison is base (3bf6d9c) 74.77% compared to head (b566193) 74.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #673      +/-   ##
==========================================
- Coverage   74.77%   74.75%   -0.03%     
==========================================
  Files         454      455       +1     
  Lines       28412    28428      +16     
  Branches      545      545              
==========================================
+ Hits        21244    21250       +6     
- Misses       7084     7095      +11     
+ Partials       84       83       -1     

Flag	Coverage Δ
frontend	77.48% <ø> (-0.14%)	⬇️
ruby-api	51.31% <ø> (-0.06%)	⬇️
ruby-backend	78.42% <ø> (+0.01%)	⬆️

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see 11 files with indirect coverage changes

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