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_en.arb

@@ -16,6 +16,13 @@
   "libraryTitle": "Library",
   "libraryCreateStudyset": "Create studyset",
   "libraryEmpty": "No studysets yet",
+  "libraryOnItsWay": "Your library is on its way from {instance}…",
+  "@libraryOnItsWay": {
+    "description": "An empty library while this device's first sync brings the person's studysets.",
+    "placeholders": {
+      "instance": {"type": "String", "example": "learn.halkyon.eu"}
+    }
+  },
   "libraryUnreadable": "Couldn't read your studysets.",
   "libraryArchivedSection": "Archived",
   "librarySearchTooltip": "Search",