first commit i guess

Service/FileService.cs

@@ -0,0 +1,25 @@
+using System.Collections.ObjectModel;
+using System.Text.Json;
+using NoteApp.Model;
+
+namespace NoteApp.Service;
+
+public class FileService {
+    private readonly string _fileName;
+    public FileService(){
+        _fileName = Path.Combine(FileSystem.AppDataDirectory, "notes.json");
+    }
+
+    public async Task<ObservableCollection<Note>> LoadNotes(){
+        if(!File.Exists(_fileName))
+            return [];
+        
+        var json = await File.ReadAllTextAsync(_fileName);
+        return JsonSerializer.Deserialize<ObservableCollection<Note>>(json) ?? [];
+    }
+
+    public async Task SaveNotes(ObservableCollection<Note> notes){
+        var json = JsonSerializer.Serialize(notes);
+        await File.WriteAllTextAsync(_fileName, json);
+    }
+}