Fix window becoming invisible when started minimized

[hluk]

Skip saving window geometry while minimized to prevent persisting invalid coordinates. Validate geometry on show to recover from previously corrupted state.

Fixes: #3440
Assisted-by: Claude (Anthropic)

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.85%. Comparing base (5ea6b73) to head (2cdd097).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/gui/geometry.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3494   +/-   ##
=======================================
  Coverage   80.85%   80.85%           
=======================================
  Files         245      245           
  Lines       31282    31285    +3     
=======================================
+ Hits        25294    25297    +3     
  Misses       5988     5988           

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