fix: resolve note todos synchronization issue between mobile and desktop

Fixed critical bug where todos added on mobile or desktop were being silently lost.

Root cause:
- Mobile and desktop were sending todos via the generic PUT /api/notes/:id endpoint
- Backend ignored the 'todos' field in this endpoint (only processes title and content)
- Users saw success messages but todos were never saved to database
- On reload, todos disappeared

Solution:
- Added dedicated todo management methods to NotesService:
  * addTodo() - POST /api/notes/:id/todos
  * updateTodo() - PUT /api/notes/todos/:todoId
  * toggleTodo() - PUT /api/notes/todos/:todoId
  * deleteTodo() - DELETE /api/notes/todos/:todoId

- Updated NoteDetailPage.tsx (mobile) to use proper API methods
- Updated Index.tsx (desktop) to use proper API methods
- Both versions now correctly persist todos to the database

Files modified:
- src/services/NotesService.ts - Added todo management methods
- src/pages/mobile/NoteDetailPage.tsx - Fixed todo handlers
- src/pages/Index.tsx - Fixed todo handlers in note detail view

src/services/NotesService.ts

@@ -219,6 +219,85 @@ class NotesService {
       return false;
     }
   }
+
+  // Todo management methods
+  async addTodo(noteId: number, text: string): Promise<Todo | null> {
+    try {
+      const response = await fetch(`/api/notes/${noteId}/todos`, {
+        method: "POST",
+        headers: AuthService.getHeaders(),
+        body: JSON.stringify({ text })
+      });
+
+      if (!response.ok) {
+        throw new Error("Erreur lors de l'ajout de la tâche");
+      }
+
+      const result = await response.json();
+      showSuccess("Tâche ajoutée");
+      return result.todo || result;
+    } catch (error) {
+      showError(error instanceof Error ? error.message : "Erreur serveur");
+      return null;
+    }
+  }
+
+  async updateTodo(todoId: number, updates: Partial<Todo>): Promise<Todo | null> {
+    try {
+      const response = await fetch(`/api/notes/todos/${todoId}`, {
+        method: "PUT",
+        headers: AuthService.getHeaders(),
+        body: JSON.stringify(updates)
+      });
+
+      if (!response.ok) {
+        throw new Error("Erreur lors de la mise à jour de la tâche");
+      }
+
+      const result = await response.json();
+      return result.todo || result;
+    } catch (error) {
+      showError(error instanceof Error ? error.message : "Erreur serveur");
+      return null;
+    }
+  }
+
+  async toggleTodo(todoId: number, completed: boolean): Promise<boolean> {
+    try {
+      const response = await fetch(`/api/notes/todos/${todoId}`, {
+        method: "PUT",
+        headers: AuthService.getHeaders(),
+        body: JSON.stringify({ completed })
+      });
+
+      if (!response.ok) {
+        throw new Error("Erreur lors de la mise à jour de la tâche");
+      }
+
+      return true;
+    } catch (error) {
+      showError(error instanceof Error ? error.message : "Erreur serveur");
+      return false;
+    }
+  }
+
+  async deleteTodo(todoId: number): Promise<boolean> {
+    try {
+      const response = await fetch(`/api/notes/todos/${todoId}`, {
+        method: "DELETE",
+        headers: AuthService.getHeaders()
+      });
+
+      if (!response.ok) {
+        throw new Error("Erreur lors de la suppression de la tâche");
+      }
+
+      return true;
+    } catch (error) {
+      showError(error instanceof Error ? error.message : "Erreur serveur");
+      return false;
+    }
+  }
 }
 
 export default new NotesService();