Add project files.

ModVersionChecker/checkers/ApiChecker.cs

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+using ModVersionChecker.data.model;
+using System;
+using System.Net.Http;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+namespace ModVersionChecker
+{
+    public class ApiChecker : IVersionChecker
+    {
+        private static readonly HttpClient _httpClient = new HttpClient();
+
+        public async Task<string> GetLatestVersion(Dictionary<string, string> paramsDict, SourceDef source)
+        {
+            if (!paramsDict.TryGetValue("url", out var url) || string.IsNullOrEmpty(url))
+            {
+                throw new ArgumentException("API URL required");
+            }
+            if (!paramsDict.TryGetValue("jsonPath", out var jsonPath) || string.IsNullOrEmpty(jsonPath))
+            {
+                throw new ArgumentException("jsonPath required");
+            }
+
+            var response = await _httpClient.GetAsync(url);
+            if (!response.IsSuccessStatusCode)
+            {
+                throw new Exception($"API error: {(int)response.StatusCode} {response.ReasonPhrase}");
+            }
+
+            var body = await response.Content.ReadAsStringAsync();
+            if (string.IsNullOrEmpty(body))
+            {
+                throw new Exception("Empty API response");
+            }
+
+            using var jsonDoc = JsonDocument.Parse(body);
+            var element = jsonDoc.RootElement;
+
+            foreach (var key in jsonPath.Split('.'))
+            {
+                if (!element.TryGetProperty(key, out var nextElement))
+                {
+                    throw new Exception($"JSON key '{key}' not found in response");
+                }
+                element = nextElement;
+            }
+
+            if (element.ValueKind != JsonValueKind.String)
+            {
+                throw new Exception($"JSON value for '{jsonPath}' is not a string");
+            }
+
+            return element.GetString()!.Trim();
+        }
+    }
+}