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/js/reader.js

@@ -674,6 +674,9 @@
     document.body.classList.add("overflow-hidden");
     applyType();
     show(firstUnread());
+    // Move focus into the scroll region so Space / PageUp-Down / arrow keys
+    // scroll the story, not the page behind it, without a click first.
+    if (scrollEl) { try { scrollEl.focus({ preventScroll: true }); } catch (e) { scrollEl.focus(); } }
   }
   function closeReader() {
     open = false;