feat(settings): how many changes wait to sync

The Sync card in Settings says how many changes made on this device
haven't reached the server yet: "3 changes wait to sync", in place of
"Everything synced" until a sync takes them, and beside a failure.

The sync engine can't tell, since the server only says what it has at
the start of each sync, so the app counts: every save asks for a sync,
and a sync that goes through takes the saves made before it started.
The count is kept in the person's folder, so it outlives the app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -1670,6 +1670,17 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get syncNow => 'Sync now';
 
+  @override
+  String syncPending(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count changes wait to sync',
+      one: '1 change waits to sync',
+    );
+    return '$_temp0';
+  }
+
   @override
   String get syncTroubleLabel => 'Sync needs attention';