feat(ui): allow copying flags across environments

[AnouarMohamed]

Summary

Adds support for copying a flag to a namespace in another environment from the flag details page.

Fixes #5382.

Changes

• Added a dedicated copy dialog with environment and namespace selectors
• Updated flag copy to read from the source environment and write using the destination environment revision
• Added UI coverage for cross-environment flag copy
• Included a small Listbox control fix as a prerequisite for dependent selections

Backward Compatibility

Existing same-environment namespace copy behavior still works.

[lamoboos223]

@AnouarMohamed

This is great we should also add the ability to populate segments as well

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.05%. Comparing base (e7645db) to head (79d517a).

Additional details and impacted files

@@           Coverage Diff           @@
##               v2    #5772   +/-   ##
=======================================
  Coverage   61.05%   61.05%           
=======================================
  Files         141      141           
  Lines       14211    14211           
=======================================
  Hits         8676     8676           
  Misses       4809     4809           
  Partials      726      726           

Flag	Coverage Δ
integrationtests	34.21% <ø> (ø)
unittests	52.38% <ø> (ø)

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