refactor: share the Date field shell and calendar frame (jscpd: ~600 duplicated lines) #623

jscpd reports the Date components as the largest duplication: DatePicker/TimeField 188 lines, DateField/TimeField 147, Calendar/RangeCalendar 122, DatePicker/DateRangePicker 13.

Extract, internal only (no public API change):

• a field shell (label, description, error, hidden input, group wrapper, trigger button) used by DateField, TimeField, DatePicker, DateRangePicker;
• a calendar frame (application root, sr-only heading, CalendarBody) used by Calendar, RangeCalendar and the two picker popovers;
• the shared popover helpers (focusOwnDay, close).

Proof: the existing Date browser tests stay green (no new behavior, so no new test).