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>

test/services/http_client_test.dart

@@ -0,0 +1,90 @@
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
+
+import 'package:deflockapp/dev_config.dart';
+import 'package:deflockapp/services/http_client.dart';
+
+void main() {
+  group('UserAgentClient', () {
+    test('adds User-Agent header to GET requests', () async {
+      String? capturedUserAgent;
+
+      final inner = MockClient((request) async {
+        capturedUserAgent = request.headers['User-Agent'];
+        return http.Response('ok', 200);
+      });
+
+      final client = UserAgentClient(inner);
+      await client.get(Uri.parse('https://example.com'));
+
+      expect(capturedUserAgent, isNotNull);
+      expect(capturedUserAgent, startsWith('DeFlock/'));
+      expect(capturedUserAgent, contains('+$kHomepageUrl'));
+      expect(capturedUserAgent, contains('contact: $kContactEmail'));
+    });
+
+    test('adds User-Agent header to POST requests', () async {
+      String? capturedUserAgent;
+
+      final inner = MockClient((request) async {
+        capturedUserAgent = request.headers['User-Agent'];
+        return http.Response('ok', 200);
+      });
+
+      final client = UserAgentClient(inner);
+      await client.post(
+        Uri.parse('https://example.com'),
+        body: json.encode({'key': 'value'}),
+      );
+
+      expect(capturedUserAgent, isNotNull);
+      expect(capturedUserAgent, startsWith('DeFlock/'));
+    });
+
+    test('preserves existing headers alongside User-Agent', () async {
+      Map<String, String>? capturedHeaders;
+
+      final inner = MockClient((request) async {
+        capturedHeaders = request.headers;
+        return http.Response('ok', 200);
+      });
+
+      final client = UserAgentClient(inner);
+      await client.get(
+        Uri.parse('https://example.com'),
+        headers: {'Authorization': 'Bearer token123'},
+      );
+
+      expect(capturedHeaders, isNotNull);
+      expect(capturedHeaders!['Authorization'], equals('Bearer token123'));
+      expect(capturedHeaders!['User-Agent'], startsWith('DeFlock/'));
+    });
+
+    test('does not overwrite a manually-set User-Agent', () async {
+      String? capturedUserAgent;
+
+      final inner = MockClient((request) async {
+        capturedUserAgent = request.headers['User-Agent'];
+        return http.Response('ok', 200);
+      });
+
+      final client = UserAgentClient(inner);
+      await client.get(
+        Uri.parse('https://example.com'),
+        headers: {'User-Agent': 'CustomAgent/1.0'},
+      );
+
+      expect(capturedUserAgent, equals('CustomAgent/1.0'));
+    });
+
+    test('static userAgent getter returns expected format', () {
+      final ua = UserAgentClient.userAgent;
+      expect(ua, startsWith('DeFlock/'));
+      expect(ua, contains('+$kHomepageUrl'));
+      expect(ua, contains('contact: $kContactEmail'));
+    });
+  });
+}