feat(chat): disallow links in messages

Local-first chat between people whose web of trust is still forming is a
natural phishing/scam vector, so messages may not contain URLs. A
conservative `containsUrl` rule (explicit scheme, www., or a common-TLD
domain — but not "3.5kg" or "12.30") gates it: the composer warns and
keeps the text for editing, and MessagesCubit.send drops any URL as a
backstop. Incoming text was already non-tappable (plain selectable text).

apps/app_seeds/test/domain/message_rules_test.dart

@@ -0,0 +1,31 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/domain/message_rules.dart';
+
+void main() {
+  group('containsUrl', () {
+    test('flags explicit schemes and www', () {
+      expect(containsUrl('look https://evil.example/login'), isTrue);
+      expect(containsUrl('http://foo'), isTrue);
+      expect(containsUrl('go to www.example.org now'), isTrue);
+    });
+
+    test('flags bare domains with a common TLD', () {
+      expect(containsUrl('see tomate.com'), isTrue);
+      expect(containsUrl('my-shop.store has it'), isTrue);
+      expect(containsUrl('grab it at bit.ly'), isFalse); // ly not in the list
+    });
+
+    test('leaves ordinary seed talk alone', () {
+      expect(containsUrl('tengo 3.5kg de tomate rosa'), isFalse);
+      expect(containsUrl('variedad F1 vs polinización abierta'), isFalse);
+      expect(containsUrl('nos vemos a las 12.30'), isFalse);
+      expect(containsUrl('semillas de calabaza, ¿cambiamos?'), isFalse);
+      expect(containsUrl(''), isFalse);
+    });
+
+    test('is case-insensitive', () {
+      expect(containsUrl('HTTPS://EXAMPLE.COM'), isTrue);
+      expect(containsUrl('Example.COM'), isTrue);
+    });
+  });
+}