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/data/default_sources.dart

@@ -185,3 +185,12 @@ const defaultSources = <NewsSource>[
     description: 'Security and malware news',
   ),
 ];
+
+/// The curated sources before the app remembered which ones it had seen.
+/// Installs from then count these as seen, so switched-off ones stay off.
+const legacySeenSourceIds = {
+  'heise', 'golem', 't3n', 'computerbase', 'spiegel-netzwelt', 'zeit-digital',
+  'netzpolitik', 'heise-security', 'heise-developer', 'caschy', 'derstandard-web',
+  'borncity', 'verge', 'arstechnica', 'techcrunch', 'hackernews', 'mit-tr', 'wired',
+  'engadget', 'theregister', 'bleepingcomputer',
+};