fix(web): add clipboard fallback for non-HTTPS contexts

[GautamKumarOffical]

Summary

Adds a clipboard fallback for non-HTTPS contexts (e.g., localhost without HTTPS).

Fixes#32664

Problem

The copy-to-clipboard button for code blocks doesn't work in opencode web when opened on localhost without HTTPS. This is because navigator.clipboard requires HTTPS in most browsers.

Solution

Added a fallback using document.execCommand('copy') when navigator.clipboard is not available:

1. Try navigator.clipboard.writeText() first (works with HTTPS)
2. If not available, create a temporary textarea, select its content, and use document.execCommand('copy')
3. Clean up the temporary textarea after copying

Testing

• Tested on localhost without HTTPS — copy button now works
• Tested on HTTPS — existing behavior unchanged
• Tested on desktop app — no regression

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found 1 potentially related PR:

fix(app): clipboard fallback so copy buttons work in non-secure contexts
#17549

This PR addresses the same issue (clipboard fallback for non-secure/non-HTTPS contexts) but targets the app package instead of web. It may have already solved this problem, so check if the solution can be reused or if this PR duplicates that work.

[github-actions]

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