refactor(ui): use Button loading prop instead of manual Spinner

Address PR #698 review feedback from Gemini Code Assist to use the
Button component's built-in loading prop instead of manually rendering
Spinner components.

The Button component already handles spinner display with correct
variant selection (foreground for default/destructive buttons, primary
for others), so this simplifies the code and aligns with component
abstractions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -86,8 +86,7 @@ export function IssueDetailPanel({
           {(() => {
             if (isValidating) {
               return (
-                <Button variant="default" size="sm" disabled>
+                <Button variant="default" size="sm" disabled loading>
-                  <Spinner size="sm" variant="foreground" className="mr-1" />
                   Validating...
                 </Button>
               );
@@ -335,15 +334,9 @@ export function IssueDetailPanel({
                       className="w-full"
                       onClick={loadMore}
                       disabled={loadingMore}
+                      loading={loadingMore}
                     >
-                      {loadingMore ? (
+                      {loadingMore ? 'Loading...' : 'Load More Comments'}
-                        <>
-                          <Spinner size="sm" className="mr-2" />
-                          Loading...
-                        </>
-                      ) : (
-                        'Load More Comments'
-                      )}
                     </Button>
                   )}
                 </div>