Links shared on social media don't show a preview — need OG tags

When we share product or blog links on Twitter/LinkedIn/Slack, they show up as plain URLs with no preview card. We're missing Open Graph meta tags.

Each page needs at minimum:
- `og:title`
- `og:description`
- `og:image` (we can use a default brand image for now)
- `og:url`

This is low effort but high impact for our social sharing.