fix: use selectedForeground for transparent background themes in dialogs

[ShamanicArts]

Fixes #9920

Fixes invisible button text in DialogConfirm, DialogAlert, and DialogHelp when using transparent background themes.

Problem

These dialog components used theme.selectedListItemText directly for button text color. For themes without an explicit selectedListItemText defined, this defaults to theme.background. On transparent background themes, this means the text color is literally transparent (alpha=0), making button labels completely invisible.

Solution

Use the selectedForeground(theme) function instead, which:

• Detects transparent backgrounds
• Calculates proper contrasting color (black or white based on primary color luminance)
• Falls back appropriately for non-transparent themes

This matches how DialogSelect already handles this case correctly.

Before:

After:

Files Changed

• packages/opencode/src/cli/cmd/tui/ui/dialog-confirm.tsx
• packages/opencode/src/cli/cmd/tui/ui/dialog-alert.tsx
• packages/opencode/src/cli/cmd/tui/ui/dialog-help.tsx

[github-actions]

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[github-actions]

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

No duplicate PRs found

[ariane-emory]

LGTM: this looks like it's consistent with fixes to a similar problem in other locations that I'd contributed in the long-since merged PRs #4572 (including a function contributed by @knanao) and #7246.