Various CSS and sizing adjustments

[philippjfr]

Various tweaks to Card, FlexBox, ReactiveHTML components.

[codecov]

Codecov Report

Merging #4466 (b714164) into main (8caa820) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4466      +/-   ##
==========================================
+ Coverage   82.20%   82.22%   +0.02%     
==========================================
  Files         243      243              
  Lines       35246    35249       +3     
==========================================
+ Hits        28974    28984      +10     
+ Misses       6272     6265       -7     

Flag	Coverage Δ
ui-tests	35.75% <66.66%> (+<0.01%)	⬆️
unitexamples-tests	74.10% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
panel/io/resources.py	81.76% <100.00%> (+0.10%)	⬆️
panel/layout/flex.py	95.83% <100.00%> (+0.18%)	⬆️
panel/io/state.py	70.63% <0.00%> (+0.17%)	⬆️
panel/reactive.py	79.31% <0.00%> (+0.40%)	⬆️
panel/io/reload.py	73.91% <0.00%> (+2.17%)	⬆️

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