Add OpenGraph/Twitter cards, generated per sin at build time

Link previews are the funnel for a "send the link" site, so give every sin
a share-optimized card.

- og/twitter meta tags on every page (poke as description, canonical URL,
  summary_large_image)
- Per-sin 1200x630 card generated at build: the poke, the before/after
  charts (uniform height so tall vconcat charts fit), severity meter,
  category, and short citations — composed with satori, rasterized with
  @resvg/resvg-js, no browser needed
- Default brand card for the homepage and canon
- Shared chart pipeline extracted to lib/renderChart.ts (used by both the
  on-page component and the card generator)
- Bundle three IBM Plex TTF weights for the rasterizer

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XPEvmMbac2fCvKXpQcovj8

package.json

@@ -14,7 +14,10 @@
     "@astrojs/mdx": "^4.2.0",
     "@fontsource/ibm-plex-mono": "^5.3.0",
     "@fontsource/ibm-plex-sans": "^5.3.0",
+    "@resvg/resvg-js": "^2.6.2",
     "astro": "^5.6.0",
+    "satori": "^0.29.0",
+    "satori-html": "^0.3.2",
     "vega": "^5.30.0",
     "vega-lite": "^5.21.0"
   }