feat: refactor HTTP client setup for Github and Forgejo services

- Centralized HTTP client configuration for `GithubService` and `ForgejoService` using `AddHttpClient`.
- Enhanced maintainability by moving Forgejo token and base URL setup into DI-configured HTTP clients.
- Fixed `forgejoInstance` validation logic and adjusted release selection to prioritize the first non-prerelease.

Program.cs

@@ -1,4 +1,5 @@
-using ForgejoReleaseSync.Services;
+using System.Net.Http.Headers;
+using ForgejoReleaseSync.Services;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 
@@ -11,12 +12,19 @@ var repositoryConfig = await RepoConfigLoader.LoadConfigAsync(
     config.RepositoryConfigFilePath);
 builder.Services.AddSingleton(repositoryConfig);
 
-var client = new HttpClient();
-client.DefaultRequestHeaders.Add("User-Agent", "ForgejoReleaseSync");
-client.DefaultRequestHeaders.Add("Accept", "application/json");
-client.Timeout = TimeSpan.FromMinutes(3);
-client.MaxResponseContentBufferSize = 1024 * 1024 * 1;
-builder.Services.AddSingleton(client);
+builder.Services.AddHttpClient<GithubService>(client =>
+{
+    client.BaseAddress = new Uri("https://api.github.com/");
+    client.DefaultRequestHeaders.UserAgent.ParseAdd("ForgejoReleaseSync");
+    client.DefaultRequestHeaders.Accept.ParseAdd("application/vnd.github+json");
+});
+
+builder.Services.AddHttpClient<ForgejoService>(client =>
+{
+    client.BaseAddress = new Uri(config.ForgejoInstacne);
+    client.DefaultRequestHeaders.UserAgent.ParseAdd("ForgejoReleaseSync");
+    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("token", config.ForgejoToken);
+});
 
 builder.Services.AddSingleton<ReleaseAssetDownloadService>();
 builder.Services.AddSingleton<GithubService>();
@@ -29,6 +37,4 @@ using var scope = app.Services.CreateScope();
 
 await scope.ServiceProvider
     .GetRequiredService<SyncService>()
-    .Sync();
-
-await app.RunAsync();
+    .Sync();