Adjust ReactiveHTML css resources for relative paths

[philippjfr]

Fixes #5775

[codecov]

Codecov Report

Merging #5779 (ee2413a) into main (f645147) will increase coverage by 0.02%.
Report is 3 commits behind head on main.
The diff coverage is 48.00%.

@@            Coverage Diff             @@
##             main    #5779      +/-   ##
==========================================
+ Coverage   73.65%   73.68%   +0.02%     
==========================================
  Files         289      290       +1     
  Lines       42127    42151      +24     
==========================================
+ Hits        31030    31058      +28     
+ Misses      11097    11093       -4     

Flag	Coverage Δ
unitexamples-tests	73.68% <48.00%> (+0.02%)	⬆️

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Files	Coverage Δ
panel/reactive.py	79.14% <100.00%> (+0.03%)	⬆️
panel/tests/ui/io/test_jupyterlite.py	33.33% <0.00%> (ø)
panel/tests/ui/io/test_resources.py	45.45% <45.45%> (ø)

... and 6 files with indirect coverage changes

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