Focus reader scroll region on open so keys scroll the story

The reader overlay opened without moving focus, so Arrow-Up/Down and
PageUp/Down scrolled the page behind it until the user clicked into the
text. Make the scroll container focusable (tabindex=-1) and focus it on
open, and drop its focus outline.

internal/web/static/css/input.css

@@ -307,6 +307,7 @@ html[data-phase="night"] {
     -webkit-overflow-scrolling: touch;
     border-radius: 1.75rem;
   }
+  .pete-reader-scroll:focus { outline: none; }
 
   .pete-reader-article {
     background: var(--card);