docs(readme): replace stale UI screenshot with current dashboard on new logo #3484

Problem

The README hero screenshot is stale. README.md:24 still points at https://cozystack.io/img/screenshot-dark.png, which shows the legacy dark UI — several releases behind the dashboard users actually get. It is the first image anyone sees on the repository page.

What is needed

Fresh dashboard captures carrying the current Cozystack logo, published to the website image path, then a one-line README swap to the new asset. Worth taking the light/dark pair while capturing so the README can use the #gh-light-mode-only / #gh-dark-mode-only split the logo lines above already use.

Why this is an issue and not the original PR

#2839 was a one-line swap to https://cozystack.io/img/carousel/1.png and was approved by Aleksei Sviridkin (@lexfrei). It never merged because of the blocker I noted on it myself: the website gallery captures predate the logo change, so pointing the README at them would ship a hero image with the old logo. That makes this a screenshot-production task, not a code change — the README edit is the trivial last step. Tracking it here; the one-line diff lives on branch docs/readme-new-ui-screenshot and is not worth keeping open as a draft PR for three months while waiting on the images.

Supersedes #2839.