fix(autotranslate): mark warmup thread as IsBackground

F9.2: PreloadCache spawned a new Thread without IsBackground, which kept
the plugin unload blocked until the warmup finished (typically
100-300 ms). Setting IsBackground=true plus a named thread matches the
pattern already used in MessageManager (F6.1) and Plugin.RetentionSweep
(F9.3) since v1.4.0.

HellionChat/Util/AutoTranslate.cs

@@ -54,15 +54,21 @@ internal static class AutoTranslate
     }
 
     // Warms the auto-translate cache on a background thread so the first
-    // message send doesn't hitch the main thread.
+    // message send doesn't hitch the main thread. IsBackground keeps plugin
+    // unload non-blocking even if the warmup is still in flight.
     internal static void PreloadCache()
     {
-        new Thread(() =>
+        var thread = new Thread(() =>
         {
             var sw = Stopwatch.StartNew();
             AllEntries();
             Plugin.Log.Debug($"Warming up auto-translate took {sw.ElapsedMilliseconds}ms");
-        }).Start();
+        })
+        {
+            IsBackground = true,
+            Name = "HellionChat-AutoTranslate-Warmup",
+        };
+        thread.Start();
     }
 
     private static List<AutoTranslateEntry> AllEntries()