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/dev_config.dart

@@ -52,6 +52,8 @@ double topPositionWithSafeArea(double baseTop, EdgeInsets safeArea) {
 // Client name for OSM uploads ("created_by" tag)
 const String kClientName = 'DeFlock';
 // Note: Version is now dynamically retrieved from VersionService
+const String kContactEmail = 'admin@stopflock.com';
+const String kHomepageUrl = 'https://deflock.org';
 
 // Upload and changeset configuration
 const Duration kUploadHttpTimeout = Duration(seconds: 30); // HTTP request timeout for uploads