fix(http): remove connectionTimeout from HttpClient creation

HttpService.createHttpClient set connectionTimeout: 30s on every HttpClient.
In the background isolate on real Android devices, DNS resolution can exceed
this when the network stack is cold, causing 'SocketException: Failed host
lookup' on every background update check. Restore main's behavior of no
timeout (null), which allows DNS to resolve at the system's pace.

lib/providers/source_provider.dart

@@ -1248,11 +1248,9 @@ class TypedSettings {
 
 class HttpService {
   static const int maxRedirects = 10;
-  static const Duration connectionTimeout = Duration(seconds: 30);
 
   HttpClient createHttpClient(bool insecure) {
     final client = HttpClient();
-    client.connectionTimeout = connectionTimeout;
     if (insecure) {
       client.badCertificateCallback =
           (X509Certificate cert, String host, int port) => true;