Fix og:image for social platform compatibility

[jpr5]

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