Add consistent User-Agent header to all HTTP clients

Create UserAgentClient (http.BaseClient wrapper) that injects a
User-Agent header into every request, reading app name and version
from VersionService and contact/homepage from dev_config.dart.

Format follows OSM tile usage policy:
  DeFlock/<version> (+https://deflock.org; contact: admin@stopflock.com)

Replaces 4 inconsistent hardcoded UA strings and adds UA to the 9
call sites that previously sent none.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/services/http_client.dart

@@ -0,0 +1,34 @@
+import 'package:http/http.dart' as http;
+
+import '../dev_config.dart';
+import 'version_service.dart';
+
+/// An [http.BaseClient] that injects a User-Agent header into every request.
+///
+/// Reads the app name and version dynamically from [VersionService] so the UA
+/// string stays in sync with pubspec.yaml without hard-coding values.
+///
+/// Uses [putIfAbsent] so a manually-set User-Agent is never overwritten.
+class UserAgentClient extends http.BaseClient {
+  final http.Client _inner;
+
+  UserAgentClient([http.Client? inner]) : _inner = inner ?? http.Client();
+
+  /// The User-Agent string sent with every request.
+  ///
+  /// Format follows OSM tile usage policy recommendations:
+  /// `AppName/version (+homepage; contact: email)`
+  static String get userAgent {
+    final vs = VersionService();
+    return '${vs.appName}/${vs.version} (+$kHomepageUrl; contact: $kContactEmail)';
+  }
+
+  @override
+  Future<http.StreamedResponse> send(http.BaseRequest request) {
+    request.headers.putIfAbsent('User-Agent', () => userAgent);
+    return _inner.send(request);
+  }
+
+  @override
+  void close() => _inner.close();
+}