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/CrematoriumStep/CrematoriumStep.tsx

@@ -149,6 +149,12 @@ export const CrematoriumStep: React.FC<CrematoriumStepProps> = ({
         Crematorium
       </Typography>
 
+      <Typography variant="body2" color="text.secondary" sx={{ mb: 4 }}>
+        {isPrePlanning
+          ? 'Review the crematorium details. You can update this later.'
+          : 'Confirm the crematorium and let us know about any preferences.'}
+      </Typography>
+
       <Box
         component="form"
         noValidate
@@ -239,7 +245,7 @@ export const CrematoriumStep: React.FC<CrematoriumStepProps> = ({
         </Box>
 
         {/* ─── Witness / attendance question ─── */}
-        <FormControl component="fieldset" sx={{ mb: 3, display: 'block' }}>
+        <FormControl component="fieldset" sx={{ mb: 4, display: 'block' }}>
           <FormLabel component="legend" sx={{ mb: 0.5 }}>
             Will anyone be following the hearse to the crematorium?
           </FormLabel>