Refactor datepicker calendar views to use `@angular/aria/grid`

[chintankavathia]

Background

The datepicker currently implements calendar-cell semantics, roving focus, and
two-dimensional keyboard navigation locally. Using @angular/aria/grid in the shared calendar
body can standardize this behavior across day, month, and year views while retaining
date-specific interactions.

Related to #1070.

Scope

• Integrate @angular/aria/grid into the shared calendar body and cell implementation.
• Use the grid primitive for roles, focus management, roving tabindex, and two-dimensional
  navigation in day, month, and year selection views.
• Remove only the manual ARIA and keyboard-management code made obsolete by the grid
  primitive.