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/models/news_source.dart

@@ -8,6 +8,19 @@ enum SourceLanguage {
   final String flag;
 }
 
+/// What a Source's articles are for; see GLOSSARY.md. Each kind has its own
+/// section in the app and its own time window.
+enum SourceKind {
+  news('News'),
+  learning('Learning'),
+  fun('Fun');
+
+  const SourceKind(this.label);
+
+  final String label;
+}
+
+/// A Source of any [kind]; the class name predates the kinds.
 class NewsSource {
   const NewsSource({
     required this.id,
@@ -15,6 +28,7 @@ class NewsSource {
     required this.feedUrl,
     required this.homepage,
     required this.language,
+    this.kind = SourceKind.news,
     this.description = '',
     this.enabledByDefault = false,
     this.isCustom = false,
@@ -26,6 +40,7 @@ class NewsSource {
   final String feedUrl;
   final String homepage;
   final SourceLanguage language;
+  final SourceKind kind;
   final String description;
   final bool enabledByDefault;
   final bool isCustom;
@@ -40,6 +55,7 @@ class NewsSource {
         'feedUrl': feedUrl,
         'homepage': homepage,
         'language': language.name,
+        'kind': kind.name,
         'description': description,
       };
 
@@ -49,6 +65,8 @@ class NewsSource {
         feedUrl: json['feedUrl'] as String,
         homepage: json['homepage'] as String? ?? '',
         language: SourceLanguage.values.byName(json['language'] as String? ?? 'en'),
+        // Custom sources saved before kinds existed are News sources.
+        kind: SourceKind.values.asNameMap()[json['kind']] ?? SourceKind.news,
         description: json['description'] as String? ?? '',
         isCustom: true,
       );