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/app/lib/l10n/en.arb

@@ -60,10 +60,17 @@
   "verificationInternalBody": "We couldn’t complete your request because of an internal error. Please try again.",
   "verificationResend": "Resend verification email",
   "verificationRequest": "Send a new verification email",
-  "verificationRetry": "Try again",
+  "verificationRetry": "Retry verification",
   "verificationBack": "Back to login",
   "verificationHint": "No email? Check your spam folder or request another below.",
   "verificationDemo": "DEMO · No real email is sent and no account is verified.",
   "verificationDemoLink": "Open demo confirmation link",
-  "verificationStep": "CONFIRM YOUR EMAIL"
+  "verificationStep": "CONFIRM YOUR EMAIL",
+  "demoSentHeading": "Demo email prepared.",
+  "demoSentBody": "No email was sent. Use the demo confirmation below to preview the next step.",
+  "demoVerifiedHeading": "Demo verification\ncomplete.",
+  "demoVerifiedBody": "You’ve previewed email confirmation. No account was verified. Return to login to explore the demo.",
+  "verificationRecipient": "Recipient",
+  "verificationChangeEmail": "Change email",
+  "demoResend": "Prepare demo email again"
 }