Batch 1: ToggleButtonGroup fixes, heading standardisation, spacing normalisation

- ToggleButtonGroup: increase label-to-options spacing (mb 1→2),
  top-align button content (flex-start), fix selected border
  CSS specificity by adding .Mui-selected in grouped selector
- Standardise all split-layout step headings from h4 to display3
  (ProvidersStep, PackagesStep, PreviewStep, VenueStep, CoffinsStep,
  CoffinDetailsStep) — matches D-A decision for brand warmth
- DateTimeStep: normalise section gaps (scheduling fieldset mb 3→4)
- CrematoriumStep: add subheading for consistency, normalise
  witness section spacing (mb 3→4)
- Fix input label clipping in PackagesStep and DateTimeStep
  (add pt: 0.5 to TextField containers for float-label clearance)

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

src/components/pages/CoffinDetailsStep/CoffinDetailsStep.tsx

@@ -254,7 +254,7 @@ export const CoffinDetailsStep: React.FC<CoffinDetailsStepProps> = ({
   // ─── Right panel: Option selectors + CTAs ───
   const optionsPanel = (
     <Box>
-      <Typography variant="h4" component="h1" sx={{ mb: 1 }} tabIndex={-1}>
+      <Typography variant="display3" component="h1" sx={{ mb: 1 }} tabIndex={-1}>
         Coffin details
       </Typography>