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/verification.dart

@@ -1,5 +1,9 @@
 import '../models/verification.dart';
 
+abstract interface class SimulationInfo {
+  bool get isSimulated;
+}
+
 abstract interface class VerificationService {
   Future<VerificationResult> send({
     required String email,
@@ -9,11 +13,13 @@ abstract interface class VerificationService {
 }
 
 /// Explicit preview tokens only. Never use this implementation in production.
-class DummyVerificationService implements VerificationService {
+class DummyVerificationService implements VerificationService, SimulationInfo {
   const DummyVerificationService({
     this.delay = const Duration(milliseconds: 600),
     this.sendFailure,
   });
+  @override
+  bool get isSimulated => true;
   final Duration delay;
   final VerificationError? sendFailure;