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/composables/useAuth.ts

@@ -1,4 +1,4 @@
-import type { AuthUser } from '~~/shared/auth'
+import { needsUserOnboarding, type AuthUser } from '~~/shared/auth'
 
 export function useAuth() {
   const user = useState<AuthUser | null>('auth:user', () => null)
@@ -8,9 +8,7 @@ export function useAuth() {
 
   const isAuthenticated = computed(() => Boolean(user.value))
   const isSuperAdmin = computed(() => user.value?.role === 'super_admin')
-  const needsOnboarding = computed(() => {
-    return Boolean(user.value && (user.value.mustChangePassword || user.value.needsPasskeySetup))
-  })
+  const needsOnboarding = computed(() => needsUserOnboarding(user.value))
 
   async function fetchSession(force = false) {
     if (loaded.value && !force) {