fix(dashboard): keep the hero skeleton until every hero source has loaded

The hero dropped its skeleton as soon as the history load resolved empty
and came back moments later with an Xtream recently-added slide (that
query waits for the favorites), moving every rail below by 316 px twice.
On J1 this read renderer.layoutShiftScoreSettled 0.235 on every macOS
iteration and was a race on the Linux runner.

The hero now stays in its skeleton while it has no slide and any of its
sources (history, favorites, Xtream recently added) is on its first load.
DashboardDataService no longer settles xtreamRecentlyAddedLoading before
the playlist inventory has loaded, where "no Xtream playlists" was
decided on a still empty inventory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changes/dashboard-hero-no-flicker.md

@@ -0,0 +1,8 @@
+---
+type: fix
+area: dashboard
+---
+
+The dashboard hero no longer disappears and comes back while the dashboard
+opens. On a profile with no watch history, the banner vanished for a moment
+before a new arrival appeared in it, and the rails below jumped up and down.