Add project files.

ModVersionChecker/managers/filesystem/AppsManager.cs

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+using ModVersionChecker.managers.interfaces;
+using System.Collections.Generic;
+using System.IO;
+using System.Text.Json;
+
+namespace ModVersionChecker.managers.filesystem
+{
+    public class AppsManager
+    {
+        private readonly string FilePath = Path.Combine(AppContext.BaseDirectory, "data", "apps.json");
+
+        public List<AppConfig> Load()
+        {
+            if (!File.Exists(FilePath))
+                return new List<AppConfig>();
+            var json = File.ReadAllText(FilePath);
+            return JsonSerializer.Deserialize<List<AppConfig>>(json) ?? new();
+        }
+
+        public void Save(List<AppConfig> apps)
+        {
+            var options = new JsonSerializerOptions { WriteIndented = true };
+            var json = JsonSerializer.Serialize(apps, options);
+            File.WriteAllText(FilePath, json);
+        }
+
+        public void Upsert(AppConfig app)
+        {
+            var apps = Load();
+            var index = apps.FindIndex(a => a.Id == app.Id);
+            if (index >= 0)
+            {
+                apps[index] = app;
+            }
+            else
+            {
+                apps.Add(app);
+            }
+            Save(apps);
+        }
+    }
+}