demo: pin colorSchema: dark so OG image renders dark, not light

slidev default is `colorSchema: auto` (= follow system). Headless Chrome
during Playwright export has no `prefers-color-scheme` set, so "auto"
resolves to light — and the build emits a light-mode OG image even though
the actual deck is normally viewed in dark mode. Pin to `dark` to match
both the OG image and the live default render.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ryan-williams

demo/starter/slides.md

@@ -1,5 +1,9 @@
 ---
 theme: seriph
+# Pin dark; the default `auto` (= follow system) renders the OG image in light
+# mode because headless Chrome has no media-preference set during the build's
+# Playwright export.
+colorSchema: dark
 title: Open-Athena/slidev
 description: A fork of sli.dev that adds direct manipulation — click any image to drag, resize, rotate, reorder, or crop, with snap-to-alignment, undo/redo, and multi-tab live edits.
 info: |