docs: close the OG frame on all four sides

The .frame used height:100% with margin:26px, making its box 52px
taller than the 630px card so the bottom border fell below the cutoff.
Set height:calc(100% - 52px) so the double-ruled frame closes cleanly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: alinearonsky