Update PostHog logo in GA post with light and dark variants

PostHog provided refreshed brand SVGs with fixed colors, so the single
currentColor file no longer fits. The quote shortcode gains an optional
logo-dark param that inlines a second SVG, and quotes.css shows one
variant per theme via PaperMod's body.dark toggle. The GA post now uses
posthog.svg in light mode and posthog-white.svg in dark mode.

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