Fix overlapping clicks with popup menu

[DennisLent]

Summary

• add ClickItem enum and overlap menu state to GUI
• detect overlapping airport and plane clicks, opening a popup menu when needed
• add helper for processing click items with unit tests

Testing

• cargo test
• cargo test -p rusty_runways_gui

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https://chatgpt.com/codex/tasks/task_e_68907eca2100832088869b2b1eb426a6

[codecov]

Codecov Report

❌ Patch coverage is 8.65385% with 95 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.74%. Comparing base (7d0464f) to head (06f8e24).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/gui/src/gui.rs	8.65%	95 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   34.35%   34.74%   +0.38%     
==========================================
  Files          21       21              
  Lines        1240     1278      +38     
==========================================
+ Hits          426      444      +18     
- Misses        814      834      +20     

Flag	Coverage Δ
rust	34.74% <8.65%> (+0.38%)	⬆️

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