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/ProvidersStep/ProvidersStep.tsx

@@ -147,7 +147,7 @@ export const ProvidersStep: React.FC<ProvidersStepProps> = ({
       }
     >
       {/* Header */}
-      <Typography variant="h4" component="h1" sx={{ mb: 0.5 }} tabIndex={-1}>
+      <Typography variant="display3" component="h1" sx={{ mb: 0.5 }} tabIndex={-1}>
         Choose a funeral provider
       </Typography>
       <Typography variant="body2" color="text.secondary" sx={{ mb: 3 }}>