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/DateTimeStep/DateTimeStep.tsx

@@ -197,6 +197,7 @@ export const DateTimeStep: React.FC<DateTimeStepProps> = ({
                 flexDirection: { xs: 'column', sm: 'row' },
                 gap: 2,
                 mb: 2,
+                pt: 0.5,
               }}
             >
               <TextField
@@ -225,7 +226,7 @@ export const DateTimeStep: React.FC<DateTimeStepProps> = ({
 
         {/* ─── Section 2: Scheduling ─── */}
         {showScheduling && (
-          <Box component="fieldset" sx={{ border: 0, m: 0, p: 0, mb: 3 }}>
+          <Box component="fieldset" sx={{ border: 0, m: 0, p: 0, mb: 4 }}>
             <Typography component="legend" variant="h5" sx={{ mb: 2.5 }}>
               {schedulingHeading}
             </Typography>