UI: Fix up mobile styling

[domyen]

Issue: #5652

What I did

• Bottom bar should have the same background as the Toolbar
• Addons panel should be scrollable
• Addons panel should extend the whole height of the viewport
• Addons panel should not have "change orientation" or "close" icons
• Canvas toolbar should not have "fullscreen"

How to test

• Load SB and resize your screen to <600px

[codecov]

Codecov Report

Merging #5709 into next will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             next    #5709      +/-   ##
==========================================
+ Coverage   33.25%   33.27%   +0.02%     
==========================================
  Files         648      648              
  Lines        9362     9366       +4     
  Branches     1355     1355              
==========================================
+ Hits         3113     3117       +4     
  Misses       5634     5634              
  Partials      615      615

Impacted Files	Coverage Δ
lib/ui/src/components/preview/preview.js	85.71% <100%> (+0.25%)	⬆️
lib/ui/src/components/panel/panel.js	50% <100%> (+2.94%)	⬆️
lib/ui/src/components/layout/mobile.js	77.14% <50%> (+1.38%)	⬆️

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