Add IntroStep page (wizard step 1) + audit fixes

IntroStep: urgency-sensitive segmentation entry point. ToggleButtonGroup
for forWhom (Myself/Someone else) with progressive disclosure revealing
hasPassedAway (Yes/No) via Collapse. Auto-sets hasPassedAway="no" when
forWhom="myself". Grief-sensitive copy adapts subheading per selection.
Pure presentation — props in, callbacks out.

Audit fixes (18/20 → 20/20):
- P1: Add <main> landmark wrapper in WizardLayout (all variants)
- P2: Wrap IntroStep fields in <form> for landmark + Enter-to-submit

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

src/components/templates/WizardLayout/WizardLayout.tsx

@@ -324,7 +324,9 @@ export const WizardLayout = React.forwardRef<HTMLDivElement, WizardLayoutProps>(
         )}
 
         {/* Main content area */}
-        <LayoutComponent secondaryPanel={secondaryPanel}>{children}</LayoutComponent>
+        <Box component="main" sx={{ display: 'flex', flexDirection: 'column', flex: 1 }}>
+          <LayoutComponent secondaryPanel={secondaryPanel}>{children}</LayoutComponent>
+        </Box>
 
         {/* Sticky help bar */}
         {!hideHelpBar && <HelpBar phone={helpPhone} />}