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/guest.ts

@@ -1,3 +1,5 @@
+import { getDefaultAuthenticatedPath } from '~~/shared/auth'
+
 export default defineNuxtRouteMiddleware(async () => {
   const auth = useAuth()
   await auth.fetchSession()
@@ -6,9 +8,5 @@ export default defineNuxtRouteMiddleware(async () => {
     return
   }
 
-  if (auth.needsOnboarding.value) {
-    return navigateTo('/security')
-  }
-
-  return navigateTo(auth.isSuperAdmin.value ? '/management/users' : '/security')
+  return navigateTo(getDefaultAuthenticatedPath(auth.user.value))
 })