[codex] Make file preview metadata copyable

[oscharko]

Summary

• Add copy actions for the opened file name and full file path in the Files preview header.
• Preserve Windows path separators when copying preview metadata.
• Cover the interaction with a regression test.

Validation

• npm run build:packages
• npm --workspace @oscharko-dev/keiko-ui run test -- src/app/components/desktop/widgets/cards/FilesWidget.test.tsx
• npm --workspace @oscharko-dev/keiko-ui run typecheck
• targeted ESLint
• targeted Prettier check
• git diff --check

Fixes #1545