fix: Improve spinner visibility on primary-colored backgrounds

Add variant prop to Spinner component to support different color contexts:
- 'primary' (default): Uses text-primary for standard backgrounds
- 'foreground': Uses text-primary-foreground for primary backgrounds
- 'muted': Uses text-muted-foreground for subtle contexts

Updated components where spinners were invisible against primary backgrounds:
- TaskProgressPanel: Active task indicators now visible
- Button: Auto-detects spinner variant based on button style
- Various dialogs and setup views using buttons with loaders

Fixes #670

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

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

apps/ui/src/components/views/board-view/dialogs/plan-approval-dialog.tsx

@@ -210,7 +210,7 @@ export function PlanApprovalDialog({
                 className="bg-green-600 hover:bg-green-700 text-white"
               >
                 {isLoading ? (
-                  <Spinner size="sm" className="mr-2" />
+                  <Spinner size="sm" variant="foreground" className="mr-2" />
                 ) : (
                   <Check className="w-4 h-4 mr-2" />
                 )}