Rename NewsSource to Source now that it models every kind (review item 3)

The class and its file (lib/models/source.dart) follow the glossary term.
Persisted JSON keys are unchanged.

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

lib/models/app_settings.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart' show ThemeMode;
 
-import 'news_source.dart';
+import 'source.dart';
 
 enum AiProvider {
   none('Off'),

lib/models/article.dart

@@ -1,4 +1,4 @@
-import 'news_source.dart';
+import 'source.dart';
 
 class Article {
   const Article({
@@ -14,7 +14,7 @@ class Article {
 
   final String title;
   final String link;
-  final NewsSource source;
+  final Source source;
   final DateTime? published;
 
   /// Short plain-text teaser from the feed (description / summary).

lib/models/source.dart

@@ -20,9 +20,9 @@ enum SourceKind {
   final String label;
 }
 
-/// A Source of any [kind]; the class name predates the kinds.
-class NewsSource {
-  const NewsSource({
+/// A Source of any [kind]; see GLOSSARY.md.
+class Source {
+  const Source({
     required this.id,
     required this.name,
     required this.feedUrl,
@@ -59,7 +59,7 @@ class NewsSource {
         'description': description,
       };
 
-  factory NewsSource.custom(Map<String, dynamic> json) => NewsSource(
+  factory Source.custom(Map<String, dynamic> json) => Source(
         id: json['id'] as String,
         name: json['name'] as String,
         feedUrl: json['feedUrl'] as String,