refactor: move theme listing logic from main to app module

[pamburus]

• Move list_themes function from main.rs to app.rs
• Extract Format trait to help.rs for better modularity
• Update dependencies and imports accordingly

[codecov]

Codecov Report

❌ Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.06%. Comparing base (2117fb4) to head (a08be93).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/app.rs	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1329      +/-   ##
==========================================
- Coverage   88.10%   88.06%   -0.04%     
==========================================
  Files          67       67              
  Lines       11608    11625      +17     
==========================================
+ Hits        10227    10238      +11     
- Misses       1381     1387       +6     

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