[bug] Copy message button does not work on web interface

[SpiritChen51]

Description

On the web interface, clicking the "Copy" button on a message does not copy the content to the clipboard. This happens because navigator.clipboard.writeText() can fail in non-secure contexts (non-HTTPS) or when the clipboard API is blocked by browser permissions.

Steps to reproduce

1. Open the OpenCode web interface
2. Click the copy button on any message
3. The button does not copy the content and no feedback is shown

Fix

Added a fallback using the traditional document.execCommand("copy") method via a temporary textarea element, which works even when the Clipboard API is unavailable. Also added a check to only update the "copied" state if the copy actually succeeded.

Operating System

All platforms

Terminal/Environment

Web browser

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Bug: All copy buttons in the web UI silently fail when accessed over HTTP (non-secure context) #17553: Same root cause (copy buttons silently fail on non-secure HTTP contexts because navigator.clipboard.writeText() requires a secure context) with the same proposed document.execCommand("copy") fallback fix.

Please review that issue and consider whether your report adds anything new. If so, feel free to clarify the distinction.