Add local card copy feedback

[VolkerChristian]

Summary

• show local copy success and failure overlays without shifting card layout
• animate successful copy from the darker hover color to a clearly lighter peak and back
• keep native and web copy behavior aligned, including reduced-motion handling
• open image thumbnails on mouse release instead of mouse press
• document and test the interaction contract

Validation

• npm test --prefix web (77 passed)
• cmake --build /home/voc/projects/drafts/CodexUI/build/Desktop_GCC-Debug --target codexui-conversation-cards-test
• ctest --test-dir /home/voc/projects/drafts/CodexUI/build/Desktop_GCC-Debug -R ^codexui-conversation-cards$ --output-on-failure
• git diff --check