Fix side margins on tablet screens in zimui

[dan-niles]

Changes:

• Fix side margins on tablet screens so that the content fills the entire screen.
• Modify playlist panel height calculation so that the panel fits screen vertically.

Before:

After:

Fix #252

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 1.62%. Comparing base (a34f5e8) to head (ba57d50).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #255   +/-   ##
=====================================
  Coverage   1.62%   1.62%           
=====================================
  Files         11      11           
  Lines       1047    1047           
  Branches     158     158           
=====================================
  Hits          17      17           
  Misses      1030    1030           

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[benoit74]

Not tested in real conditions, but it looks coherent, so LGTM, I assume you tested it significantly at various resolutions

We will need anyway a throughout review of look and feel at various screen sizes once new UI is ready.