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/atoms/ToggleButtonGroup/ToggleButtonGroup.tsx

@@ -114,7 +114,7 @@ export const ToggleButtonGroup = React.forwardRef<HTMLFieldSetElement, ToggleBut
               color: 'text.primary',
               fontWeight: 600,
               fontSize: '1rem',
-              mb: 1,
+              mb: 2,
               '&.Mui-focused': { color: 'text.primary' },
               '&.Mui-error': { color: 'text.primary' },
             }}
@@ -142,6 +142,9 @@ export const ToggleButtonGroup = React.forwardRef<HTMLFieldSetElement, ToggleBut
                 borderColor: 'var(--fa-color-neutral-200)',
                 marginLeft: 0,
               },
+              '&.Mui-selected': {
+                borderColor: 'primary.main',
+              },
             },
           }}
           {...groupProps}
@@ -155,7 +158,7 @@ export const ToggleButtonGroup = React.forwardRef<HTMLFieldSetElement, ToggleBut
                 flex: 1,
                 textTransform: 'none',
                 flexDirection: 'column',
-                alignItems: 'center',
+                alignItems: 'flex-start',
                 gap: 0.5,
                 py: option.description ? 2 : 1.5,
                 px: 3,