Website: explicitly size demo image on home page

Prevent re-layouts when the image finally loads.

Author: strager

website/public/index.ejs.html

@@ -158,12 +158,14 @@ <h1>quick-lint-js</h1>
       <p><a href="demo/">Try it out in your browser.</a></p>
 
       <figure>
-        <picture style="width: 100%">
+        <picture style="width: 100%; height: auto" width="1324" height="526">
           <source srcset="vscode-demo.webp" type="image/webp" />
           <img
             src="vscode-demo.png"
-            style="width: 100%"
+            style="width: 100%; height: auto"
             alt="quick-lint-js running in Visual Studio Code"
+            width="1324"
+            height="526"
           />
         </picture>
       </figure>