feat(library): an empty library says it is on its way during the first sync

A device's first sync brings a person's whole library at its end, so an
empty library said "No studysets yet" until then. Until a sync has gone
through on this device for this person, it now says "Your library is on
its way from <instance>…" with a spinner, unless the sync failed. Whether
one has is kept beside the pending count in sync_pending.json (a file
from before counts as synced), read as the person's data opens, and
carried in SyncStatus.syncedHere.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -42,6 +42,11 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get libraryEmpty => 'Noch keine Lernsets';
 
+  @override
+  String libraryOnItsWay(String instance) {
+    return 'Deine Bibliothek wird von $instance geladen …';
+  }
+
   @override
   String get libraryUnreadable =>
       'Deine Lernsets konnten nicht gelesen werden.';