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/setup-view/steps/cursor-setup-step.tsx

@@ -318,7 +318,7 @@ export function CursorSetupStep({ onNext, onBack, onSkip }: CursorSetupStepProps
                 >
                   {isLoggingIn ? (
                     <>
-                      <Spinner size="sm" className="mr-2" />
+                      <Spinner size="sm" variant="foreground" className="mr-2" />
                       Waiting for login...
                     </>
                   ) : (