web: keep the catalogue on one screen

Pin the thumbnail strip to the bottom edge of the window, let the wall
of thumbs scroll inside the stage with its own scrollbar, and letterbox
the preview so the frame fits the screen instead of growing the page.

docker/frontend/src/Library.tsx

@@ -217,7 +217,7 @@ export function Library() {
     );
 
   return (
-    <div className="adm">
+    <div className="adm lib-root">
       <header className="adm-bar">
         <a className="adm-brand" href="/">
           <img src="/assets/RecipesCamIcon.png" alt="" width="26" height="26" />