Rewrite datetime_accessor playground to interactive template format

Convert all static <pre><code> blocks to interactive playground-block
structure with textarea editors, matching the standard playground
template (CSS variables, loading overlay, playground-runtime.js).

Removed inline <script> runtime, TypeScript-only syntax (as casts,
type annotations), and made all 8 code blocks self-contained.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot