feat: add node deep link handling to DeepLinkService

Adds `deflockapp://node?id=<nodeId>` deep link support: parses the node
ID, fetches the OSM node via the public API, and delivers it via an
`onNodeDeepLink` callback for HomeScreen to register. Includes URL
parsing unit tests.

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

test/deep_link_service_test.dart

@@ -0,0 +1,26 @@
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('Node deep link URL parsing', () {
+    test('extracts node ID from valid link', () {
+      final uri = Uri.parse('deflockapp://node?id=1234567890');
+      expect(uri.host, 'node');
+      expect(uri.queryParameters['id'], '1234567890');
+    });
+
+    test('returns null for missing id param', () {
+      final uri = Uri.parse('deflockapp://node');
+      expect(uri.queryParameters['id'], isNull);
+    });
+
+    test('returns null for empty id param', () {
+      final uri = Uri.parse('deflockapp://node?id=');
+      expect(uri.queryParameters['id'], '');
+    });
+
+    test('returns null for non-numeric id', () {
+      final uri = Uri.parse('deflockapp://node?id=abc');
+      expect(int.tryParse(uri.queryParameters['id'] ?? ''), isNull);
+    });
+  });
+}