fix: GitHubUpdateChecker JSON property naming mismatch

GitHub REST API uses snake_case (tag_name, html_url, browser_download_url)
but the deserializer used JsonNamingPolicy.CamelCase which expected
tagName/htmlUrl. All properties deserialized to null, release.TagName
was empty, and CheckAsync always returned IsUpdateAvailable=false — the
update banner has never actually fired since F-008 added the checker.

Replace the policy with explicit JsonPropertyName attributes on each
property in GitHubRelease and GitHubAsset records. Verified against the
live API: latest release tag is now parsed and IsUpdateAvailable is
true when the installed version is older.

Note: users still running pre-fix builds will not see the banner; one
manual MSI install of the next release is required to roll the fix out.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: firejeff01

src/Tender.Desktop/Services/GitHubUpdateChecker.cs

@@ -1,6 +1,7 @@
 using System.Net.Http;
 using System.Reflection;
 using System.Text.Json;
+using System.Text.Json.Serialization;
 
 namespace Tender.Desktop.Services;
 
@@ -10,10 +11,10 @@ public sealed class GitHubUpdateChecker : IUpdateChecker
     private const string Owner = "firejeff01";
     private const string Repo = "tender";
 
-    private static readonly JsonSerializerOptions JsonOptions = new()
-    {
-        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
-    };
+    // GitHub REST API 用 snake_case（tag_name、html_url、browser_download_url）。
+    // 早期 .CamelCase 是 bug：對應出來的 JSON key 是 tagName/htmlUrl，全部反序列化成 null，
+    // 導致 update checker 一直回傳「無更新」。改用顯式 JsonPropertyName 屬性對映。
+    private static readonly JsonSerializerOptions JsonOptions = new();
 
     public async Task<UpdateInfo> CheckAsync(CancellationToken ct = default)
     {
@@ -87,16 +88,28 @@ private static Version GetCurrentVersion()
 
     private sealed record GitHubRelease
     {
+        [JsonPropertyName("tag_name")]
         public string? TagName { get; init; }
+
+        [JsonPropertyName("name")]
         public string? Name { get; init; }
+
+        [JsonPropertyName("body")]
         public string? Body { get; init; }
+
+        [JsonPropertyName("html_url")]
         public string? HtmlUrl { get; init; }
+
+        [JsonPropertyName("assets")]
         public List<GitHubAsset>? Assets { get; init; }
     }
 
     private sealed record GitHubAsset
     {
+        [JsonPropertyName("name")]
         public string? Name { get; init; }
+
+        [JsonPropertyName("browser_download_url")]
         public string? BrowserDownloadUrl { get; init; }
     }
 }