Add global app banners and integrate into layout

Introduce a global app banners store (GUI/src/stores/appBanners.ts) and switch authentication error/notification flows to use it. GUI/src/Layout.vue now consumes the banner store, replaces the single snackbar with a stacked, dismissible banner UI (styles, transitions and z-index included), and adds navigateToBrandTarget() to route the brand button based on auth state. GUI/src/routes/authentication/Login.vue was updated to push errors to the new banner store and remove the local alert. Minor route adjustments in GUI/src/plugins/routesLayout.ts: rename 'Dash' to 'Dashboard' and change Login visibility to Unauthenticated. codexInfo.md updated to document these UI/behavior changes.

GUI/src/routes/authentication/Login.vue

@@ -2,12 +2,13 @@
 import { computed, ref } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { AuthRequestError, login } from '@/services/authSession'
+import { useAppBannersStore } from '@/stores/appBanners'
 
 const showPassword = ref(false)
 const userName = ref('')
 const password = ref('')
 const isSubmitting = ref(false)
-const errorMessage = ref('')
+const appBannersStore = useAppBannersStore()
 
 const route = useRoute()
 const router = useRouter()
@@ -27,12 +28,11 @@ const redirectPath = computed(() => {
 
 async function handleSubmit() {
   if (submitDisabled.value) {
-    errorMessage.value = 'Bitte Benutzername und Passwort eingeben.'
+    appBannersStore.pushError('Bitte Benutzername und Passwort eingeben.', 'Anmeldung fehlgeschlagen')
     return
   }
 
   isSubmitting.value = true
-  errorMessage.value = ''
 
   try {
     await login({
@@ -43,9 +43,12 @@ async function handleSubmit() {
     await router.replace(redirectPath.value)
   } catch (error) {
     if (error instanceof AuthRequestError) {
-      errorMessage.value = error.message
+      appBannersStore.pushError(error.message, 'Anmeldung fehlgeschlagen')
     } else {
-      errorMessage.value = 'Anmeldung fehlgeschlagen. Bitte versuche es erneut.'
+      appBannersStore.pushError(
+        'Anmeldung fehlgeschlagen. Bitte versuche es erneut.',
+        'Anmeldung fehlgeschlagen',
+      )
     }
   } finally {
     isSubmitting.value = false
@@ -85,16 +88,6 @@ async function handleSubmit() {
           <p>Melde dich mit deinem bestehenden Konto an.</p>
         </div>
 
-        <v-alert
-          v-if="errorMessage"
-          type="error"
-          variant="tonal"
-          density="comfortable"
-          border="start"
-        >
-          {{ errorMessage }}
-        </v-alert>
-
         <v-text-field
           v-model="userName"
           label="Benutzername"