fix: use distinct sidebar icon for mobile sidebar toggle to avoid duplicate icon with WindowsAppMenu

[cmw-creator]

Issue for this PR

Closes #31115

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

On Windows/Linux, when the window is narrowed below the xl breakpoint, the WindowsAppMenu button (opening the application menu) and the mobile sidebar toggle button (toggling the sidebar) both show a menu (hamburger) icon. These are two different buttons with different functions but identical icons.

At full screen (xl+), the mobile sidebar toggle is hidden via xl:hidden, so only the WindowsAppMenu's hamburger icon is visible — no conflict. When the window narrows to < xl, the mobile sidebar toggle becomes visible and both buttons show the same icon.

The fix changes the mobile sidebar toggle's icon from "menu" to "sidebar" (a layout/panel icon), which matches the desktop sidebar toggle's icon used at larger breakpoints. This makes the two buttons visually distinguishable.

How did you verify your code works?

Verified the icon name sidebar exists in the legacy icon set (packages/ui/src/components/icon.tsx line 60). The desktop sidebar toggle (line 576) already uses this same icon name successfully. Only icon name changed; no structural or behavioral changes.

Screenshots / recordings

N/A — unable to capture the opencode UI in my environment, but the fix is a two-character icon name change with no logic modification.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍