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/pages/IntroStep/index.ts

@@ -0,0 +1,2 @@
+export { default } from './IntroStep';
+export * from './IntroStep';