* feat(ui): add copy to clipboard functionality for user messages with localization support
* fix(web): use setTimeout instead of window.setTimeout for correct Timeout type
* docs: add copy prompt button screenshot for PR #1669
* docs: add copy button hover screenshot for PR #1669
* docs: add copy button copied state screenshot for PR #1669
* fix(ui): reset button border/background on copy prompt button
The .user-copy-btn inherited border and background from the base
button CSS, rendering as a bordered gray box instead of a clean icon
overlay. This was especially visible in the Electron desktop app.
Add border: none and background: none to the button, and a subtle
hover background for feedback.
