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_analytics/pubspec.yaml

@@ -0,0 +1,9 @@
+name: sxchnge_analytics
+description: Privacy-minimal journey analytics contracts and in-memory dummy service.
+publish_to: none
+version: 0.1.0
+environment:
+  sdk: '>=3.13.0 <4.0.0'
+dev_dependencies:
+  flutter_test:
+    sdk: flutter