feat(frontend): module Articles complet (liste, création, modification, suppression)

- Article model (MP/PSF/PF, stock, seuil alerte)
- ArticleService : fetchAll, fetchByType, create, update, delete
- ArticleProvider : filtrage par type + recherche temps réel
- ArticlesScreen : liste, chips filtres, barre recherche, alertes stock
- ArticleFormScreen : formulaire création/édition avec validation
- Rapport PFE : section 5.6 module Articles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/services/article_service.dart

@@ -0,0 +1,28 @@
+import 'api_client.dart';
+import '../models/article.dart';
+
+class ArticleService {
+  static Future<List<Article>> fetchAll() async {
+    final res = await ApiClient.instance.get('/articles');
+    return (res.data as List).map((e) => Article.fromJson(e)).toList();
+  }
+
+  static Future<List<Article>> fetchByType(String type) async {
+    final res = await ApiClient.instance.get('/articles/type/$type');
+    return (res.data as List).map((e) => Article.fromJson(e)).toList();
+  }
+
+  static Future<Article> create(Article article) async {
+    final res = await ApiClient.instance.post('/articles', data: article.toJson());
+    return Article.fromJson(res.data);
+  }
+
+  static Future<Article> update(int id, Article article) async {
+    final res = await ApiClient.instance.put('/articles/$id', data: article.toJson());
+    return Article.fromJson(res.data);
+  }
+
+  static Future<void> delete(int id) async {
+    await ApiClient.instance.delete('/articles/$id');
+  }
+}