refactor: centralize validation, error handling, and formatting logic

Extract shared auth logic and validation rules to shared/auth.ts
Introduce utility functions for HTTP errors and user input parsing
Standardize error messages and date formatting across the app

app/middleware/auth.ts

@@ -1,3 +1,5 @@
+import { needsUserOnboarding } from '~~/shared/auth'
+
 export default defineNuxtRouteMiddleware(async (to) => {
   const auth = useAuth()
   await auth.fetchSession()
@@ -6,7 +8,7 @@ export default defineNuxtRouteMiddleware(async (to) => {
     return navigateTo('/login')
   }
 
-  if (to.path !== '/security' && auth.needsOnboarding.value) {
+  if (to.path !== '/security' && needsUserOnboarding(auth.user.value)) {
     return navigateTo('/security')
   }
 })