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/test/support/test_dependencies.dart

@@ -175,6 +175,9 @@ Dependencies testDependencies({
   DiscussionGateway? discussionGateway,
   SyncOnce? syncOnce,
   PendingChangesStore? pendingChanges,
+  // A device that synced before, as most are; false for one whose first
+  // sync is still to go through.
+  bool syncedBefore = true,
   OpenPerson? openPerson,
   AssistantHandOff? assistantHandOff,
   WebSearch? webSearch,
@@ -237,6 +240,7 @@ Dependencies testDependencies({
           final sync = SyncService(
             syncOnce ?? ({required onMerge}) async {},
             pendingChanges: pendingChanges,
+            syncedBefore: syncedBefore,
           );
           if (syncOnce != null) {
             sync.syncNow();