Add source kinds and the Learn tab with Learning sources (#2)

Every Source now has a kind: News, Learning or Fun. Curated Sources default
to News, and custom Sources persist their kind (ones saved before default
to News). Each kind has its own time window: News follows the 12/24/48 h
setting, Learning reaches back 14 days, Fun 3 days.

A new Learn tab between Briefing and Sources lists Learning articles with
the same paid/sponsored filters and reader view as Today. It is built from
a list of kinds so a Fun section can follow. Today and the Briefing only
use News. ByteByteGo is the first Learning source, on by default.

The home shell names its tabs instead of using hard-coded indexes.

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

lib/data/default_sources.dart

@@ -184,6 +184,18 @@ const defaultSources = <NewsSource>[
     language: SourceLanguage.en,
     description: 'Security and malware news',
   ),
+
+  // --- Learning -----------------------------------------------------------
+  NewsSource(
+    id: 'bytebytego',
+    name: 'ByteByteGo',
+    feedUrl: 'https://blog.bytebytego.com/feed',
+    homepage: 'https://blog.bytebytego.com',
+    language: SourceLanguage.en,
+    kind: SourceKind.learning,
+    description: 'System design explained with diagrams',
+    enabledByDefault: true,
+  ),
 ];
 
 /// The curated sources before the app remembered which ones it had seen.