Stax - Fix missing SCREEN_SIZE_WALLET flag

[nroggeman-ledger]

Description

This PR fixes a missing SCREEN_SIZE_WALLET flag for Stax, leading to a wrong SCREEN_WIDTH when building apps, and
wrong screen building

Changes include

• [*] Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (570aaa6) 60.42% compared to head (52a6bb4) 60.42%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #506   +/-   ##
=======================================
  Coverage   60.42%   60.42%           
=======================================
  Files          12       12           
  Lines        1660     1660           
=======================================
  Hits         1003     1003           
  Misses        657      657           

Flag	Coverage Δ
unittests	60.42% <ø> (ø)

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