Fix og:image for social platform compatibility (#14)

## Summary
- Added PNG version of og-image (converted from SVG at 1200x630)
- Updated og:image and twitter:image meta tags to reference PNG instead
of SVG
- Most social platforms (Twitter/X, Slack, Discord, LinkedIn, iMessage)
don't render SVG og:images

## Test plan
- Share https://vscode.copilotkit.dev on Slack/Twitter and verify the
preview card renders

Author: jpr5

docs/index.html

@@ -24,7 +24,7 @@
       property="og:description"
       content="Preview generative UI, explore 28 hooks, and inspect AG-UI protocol streams — all without leaving VS Code."
     />
-    <meta property="og:image" content="https://vscode.copilotkit.dev/og-image.svg" />
+    <meta property="og:image" content="https://vscode.copilotkit.dev/og-image.png" />
     <meta property="og:image:width" content="1200" />
     <meta property="og:image:height" content="630" />
     <meta name="twitter:card" content="summary_large_image" />
@@ -33,7 +33,7 @@
       name="twitter:description"
       content="Preview generative UI, explore 28 hooks, and inspect AG-UI protocol streams — all without leaving VS Code."
     />
-    <meta name="twitter:image" content="https://vscode.copilotkit.dev/og-image.svg" />
+    <meta name="twitter:image" content="https://vscode.copilotkit.dev/og-image.png" />
 
     <link rel="icon" type="image/svg+xml" href="favicon.svg" />