fix(panes): the dock is born empty, so say so

panes.css hides an empty dock (.fb-pane-dock.is-empty { display: none }), but
the element was created without the class — so between creation and the first
card it was a visible-to-CSS, announced-to-screen-readers role="region"
landmark containing nothing.

Harmless in practice today (the dock is created lazily, on the same tick as the
card that prompted it), but the CSS contract should hold from first paint rather
than from the first _syncEmpty(), and any future caller of dock() gets the right
thing for free.

Found by CodeRabbit on #928.

Signed-off-by: topkoa <topkoa@gmail.com>

static/panes/pane-dock.js

@@ -33,7 +33,11 @@
         if (!dockEl) {
             dockEl = document.createElement('div');
             dockEl.id = 'fb-pane-dock';
-            dockEl.className = 'fb-pane-dock';
+            // `is-empty` from the start: panes.css hides an empty dock, and a dock
+            // born without the class is a visible-to-CSS, announced-to-screen-readers
+            // `role="region"` landmark with nothing in it until the first card
+            // arrives. Born empty, because it is.
+            dockEl.className = 'fb-pane-dock is-empty';
             dockEl.setAttribute('role', 'region');
             dockEl.setAttribute('aria-label', 'Panes');
             document.body.appendChild(dockEl);