Revert DataGridCell default font size to 1rem

[edda]

Problem

PR #1710 reduced the default font size of DataGridCell from 1rem to 0.875rem. This change was made the default without sufficient consideration for existing consumers, who now see a smaller font in all DataGrids without opting in.

Acceptance criteria

• DataGridCell renders at the previous default font size (1rem) again.
• A smaller, opt-in size may be reintroduced later as a configurable option, but is out of scope for this issue.