CartButton molecule + progress bar on all wizard layouts

- New CartButton molecule: outlined pill trigger with receipt icon,
  "Your Plan" label + formatted total in brand colour. Click opens
  DialogShell with items grouped by section via LineItem, total row,
  empty state. Mobile collapses to icon + price.
- WizardLayout: remove STEPPER_VARIANTS whitelist — stepper bar now
  renders on any layout variant when progressStepper/runningTotal props
  are provided (StepperBar already returns null when both empty)
- Thread progressStepper + runningTotal props to DateTimeStep, VenueStep,
  SummaryStep, PaymentStep (joins 8 pages that already had them)

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

src/components/pages/SummaryStep/SummaryStep.tsx

@@ -101,6 +101,8 @@ export interface SummaryStepProps {
   navigation?: React.ReactNode;
   /** Progress stepper */
   progressStepper?: React.ReactNode;
+  /** Running total widget (e.g. CartButton) */
+  runningTotal?: React.ReactNode;
   /** Hide the help bar */
   hideHelpBar?: boolean;
   /** MUI sx prop */
@@ -287,6 +289,7 @@ export const SummaryStep: React.FC<SummaryStepProps> = ({
   isPrePlanning = false,
   navigation,
   progressStepper,
+  runningTotal,
   hideHelpBar,
   sx,
 }) => {
@@ -297,6 +300,7 @@ export const SummaryStep: React.FC<SummaryStepProps> = ({
       variant="centered-form"
       navigation={navigation}
       progressStepper={progressStepper}
+      runningTotal={runningTotal}
       showBackLink={!!onBack}
       backLabel="Back"
       onBack={onBack}