improvement(branding): dark og image matching landing surface

Swap the OG asset from a white-background wordmark to a dark variant
(#1c1c1c — the canonical --landing-bg surface color) with the brand
wordmark on top. Reads better in dark-mode chat clients (Slack, iMessage,
Discord) and brings the marketing card aesthetic in line with the
docs.sim.ai cards.

Asset only — metadata and Twitter card paths are unchanged, so the
existing OG/Twitter wiring picks up the new image automatically once
caches refresh.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: waleedlatif1