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/layouts/default.vue

@@ -305,6 +305,8 @@
 </template>
 
 <script lang="ts" setup>
+import { getErrorMessage } from '../utils/errors'
+
 interface SystemMenuItem {
   label: string
   to: string
@@ -405,7 +407,7 @@ async function logout() {
   } catch (error: any) {
     toast.add({
       title: 'Logout failed',
-      description: error?.data?.statusMessage || 'Unable to end the current session.',
+      description: getErrorMessage(error, 'Unable to end the current session.'),
       color: 'error',
       icon: 'i-lucide-circle-alert'
     })