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/uploader.dart

@@ -5,6 +5,7 @@ import 'package:http/http.dart' as http;
 import '../models/pending_upload.dart';
 import '../dev_config.dart';
 import '../state/settings_state.dart';
+import 'http_client.dart';
 import 'version_service.dart';
 
 class UploadResult {
@@ -348,6 +349,7 @@ class Uploader {
   Map<String, String> get _headers => {
         'Authorization': 'Bearer $accessToken',
         'Content-Type': 'text/xml',
+        'User-Agent': UserAgentClient.userAgent,
       };
 
   /// Sanitize text for safe inclusion in XML attributes and content