Update social/OG share image to the new Cozystack logo (#554)

## Summary

Drops the refreshed Cozystack brand mark (icon + `COZYSTACK` wordmark)
onto the existing social / Open Graph share image.

- Same `1200×630` layout, same dark `#1e1e1e` background, same
circuit-pattern decoration, same `Free Cloud Platform based on
Kubernetes` tagline.
- Only the logo in the center changes.

## Context

Follow-up to #551 which updated the in-site logo and favicons but landed
before the share image was refreshed.

## Test plan

- [ ] Validate the share image via a social-preview debugger (X/Twitter,
LinkedIn, Facebook OG).

Author: kvaps