fix(readme): use picture element for dark/light mode card embed

The #gh-light-mode-only / #gh-dark-mode-only fragment approach doesn't
work with GitHub's Camo proxy — the proxy rewrites image URLs, stripping
the fragment from the rendered src attribute so GitHub's CSS selector
can't match it. Both cards showed stacked simultaneously.

Replace with the official GitHub-documented <picture> + prefers-color-scheme
approach, consistent with the OSS Insight and Star History widgets already
in this README.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: castrojo

README.md

@@ -27,8 +27,12 @@ Bluefin's mission is to provide a robust, cloud-native desktop operating system
 
 ## Releases
 
-[![Bluefin](https://docs.projectbluefin.io/img/cards/bluefin-light.png#gh-light-mode-only)](https://docs.projectbluefin.io/changelogs)
-[![Bluefin](https://docs.projectbluefin.io/img/cards/bluefin-dark.png#gh-dark-mode-only)](https://docs.projectbluefin.io/changelogs)
+<a href="https://docs.projectbluefin.io/changelogs">
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="https://docs.projectbluefin.io/img/cards/bluefin-dark.png">
+    <img src="https://docs.projectbluefin.io/img/cards/bluefin-light.png" alt="Bluefin" width="800">
+  </picture>
+</a>
 
 ## Communications