refactor(NavHeader.astro): ⚡ Optimise the header background image size and resolution

Moved to using Astro's `getImage()` function to serve the image as an avif image wich was been scaled down so it's isn't larger resolution than it need to be

.vscode/settings.json

@@ -2,6 +2,7 @@
     "conventionalCommits.scopes": [
         "FriendLink.astro",
         "astro.config.mjs",
-        ".vscode"
+        ".vscode",
+        "NavHeader.astro"
     ]
 }