Promote PickerShell from action-row-only to a canonical 4-zone shell
(Root → Header / Body / Footer) shared by every composite picker so
date/time/color popovers get the same visual contract for free.
- Add `<PickerShell.Root>` flex-column wrapper (emits data-picker-shell,
data-mode) plus `<PickerShell.Header>` and `<PickerShell.Body>` zones
matching the existing Footer's spacing + border tokens.
- Auto-wrap all 5 *-content.svelte popovers (date / date-range / time /
time-range / color) in `<PickerShell.Root>` so consumers only compose
Body + Footer.
- Refresh all 5 demos to use `<PickerShell.Body>` around picker content;
date-picker demo gains a `showShellHeader` toggle showcasing the new
Header zone.
- CSS scales section gap + header spacing across xs/sm/md/lg picker
sizes via [data-popover-content][data-picker-size] scopes, in sync
with the existing Footer scale.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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