Switch newly added default Sources on once (#1)

The app now remembers which curated Sources it has seen. A Source that is on
by default is enabled the first time it appears, also on installs with saved
toggles; Sources seen before keep the user's choice. Installs from before this
change count the current catalog as seen.

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

lib/services/storage.dart

@@ -74,6 +74,13 @@ class Storage {
   Future<void> saveEnabledSourceIds(Set<String> ids) =>
       _prefs.setStringList('enabled_sources', ids.toList());
 
+  /// Curated sources the app has already offered, so each one that is on by
+  /// default switches on only once. Null means "never saved".
+  Set<String>? loadSeenSourceIds() => _prefs.getStringList('seen_sources')?.toSet();
+
+  Future<void> saveSeenSourceIds(Set<String> ids) =>
+      _prefs.setStringList('seen_sources', ids.toList());
+
   List<NewsSource> loadCustomSources() {
     final raw = _prefs.getString('custom_sources');
     if (raw == null) return [];