fix: support wide strings for app description and usage

[ThunderKhan]

Closes #1130.

Summary

Adds support for wide strings in the CLI::App description constructor and usage() setter.

Changes

• Add a std::wstring overload for the CLI::App description constructor.
• Add a std::wstring overload for App::usage().
• Convert wide strings through CLI11's existing narrow() helper and delegate to the existing std::string implementations.
• Add a regression test covering wide string literals with non-ASCII Unicode characters.

Testing

• Added a regression test reproducing the previously unsupported wide-string calls.
• Verified the regression test fails to compile without the new overloads.
• Built successfully with C++11.
• Ran the full test suite: 83/83 tests passed.
• Ran prek successfully.
• Ran git diff --check successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (75933a4) to head (a476824).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1439   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         5234      5236    +2     
  Branches      1132      1132           
=========================================
+ Hits          5234      5236    +2     

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