fix(ui): fix spinner visibility in github issue validation button

The spinner component in the GitHub issue validation button was blended
into the button's primary background color, making it invisible. This was
caused by the spinner using the default 'primary' variant which applies
text-primary color, matching the button's background.

Changed the spinner to use the 'foreground' variant which applies
text-primary-foreground for proper contrast against the primary background.
This follows the existing pattern already implemented in the worktree panel
components.

Fixes #697

apps/ui/src/components/views/github-issues-view/components/issue-detail-panel.tsx

@@ -87,7 +87,7 @@ export function IssueDetailPanel({
             if (isValidating) {
               return (
                 <Button variant="default" size="sm" disabled>
-                  <Spinner size="sm" className="mr-1" />
+                  <Spinner size="sm" variant="foreground" className="mr-1" />
                   Validating...
                 </Button>
               );