Progress bar layout refinement + step label readability

- StepperBar: stepper centred at 700px max-width, cart hugs right edge
- StepIndicator: bump desktop label fontSize to 0.875rem for readability
- DateTimeStep story: demo progress bar + cart in context

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/molecules/StepIndicator/StepIndicator.tsx

@@ -96,7 +96,7 @@ export const StepIndicator = React.forwardRef<HTMLDivElement, StepIndicatorProps
                   whiteSpace: 'nowrap',
                   overflow: 'hidden',
                   textOverflow: 'ellipsis',
-                  fontSize: { xs: '0.6875rem', sm: undefined },
+                  fontSize: { xs: '0.6875rem', sm: '0.875rem' },
                 }}
               >
                 {step.label}