feat(dashboard): currently playing and pile of shame tiles

Pile of Shame shows the backlog count and share, a status bar and a random
backlog pick. Both tiles show the first-sync text while the initial sync runs;
a failed summary load shows a retry button.

src/utils/backlog.js

@@ -19,3 +19,6 @@ export const libraryQuery = ({page, size, sort, status, q}) => {
     if (q?.trim()) params.set('q', q.trim());
     return params.toString();
 };
+
+export const backlogShare = (summary) =>
+    summary?.total ? Math.round(((summary.byStatus?.BACKLOG ?? 0) / summary.total) * 100) : 0;