Fix onboarding UX and add replaceable journey analytics

Make recipient changes explicit, prioritize verification retry, and label simulated outcomes prominently. Add shared typed analytics with bounded dummy capture, correlated service events and documented PostHog adapter contract. Validate privacy, retries, abandonment, keyboard recovery and accessibility. Related to #12.

frontend/packages/sxchnge_auth/lib/src/services/dummy.dart

@@ -1,9 +1,12 @@
 import '../models/auth.dart';
 import 'contracts.dart';
+import 'verification.dart';
 
 /// Deterministic local simulation. No network, storage, credentials or sessions
 /// are retained. Configure failures explicitly rather than using magic emails.
-class DummyLoginService implements LoginService {
+class DummyLoginService implements LoginService, SimulationInfo {
+  @override
+  bool get isSimulated => true;
   const DummyLoginService({
     this.delay = const Duration(milliseconds: 600),
     this.failure,
@@ -19,7 +22,9 @@ class DummyLoginService implements LoginService {
   }
 }
 
-class DummySignupService implements SignupService {
+class DummySignupService implements SignupService, SimulationInfo {
+  @override
+  bool get isSimulated => true;
   const DummySignupService({
     this.delay = const Duration(milliseconds: 600),
     this.failure,