feat(presentation): add selected-tab navigation provider

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/presentation/providers/navigation_provider.dart

@@ -0,0 +1,13 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+/// Index de l'onglet sélectionné dans la navigation principale.
+/// Remplace le hack findAncestorStateOfType<MainPageState>.
+class SelectedTabNotifier extends Notifier<int> {
+  @override
+  int build() => 0;
+
+  void set(int index) => state = index;
+}
+
+final selectedTabProvider =
+    NotifierProvider<SelectedTabNotifier, int>(SelectedTabNotifier.new);