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/plugins/routesLayout.ts

@@ -52,7 +52,7 @@ export const routes: LayoutRoute[] = [
   },
   {
     path: '/',
-    name: 'Dash',
+    name: 'Dashboard',
     description: 'Geschützter Bereich für dein Konto',
     icon: 'mdi-view-dashboard-outline',
     visible: Visibility.Authenticated,
@@ -70,7 +70,7 @@ export const routes: LayoutRoute[] = [
     name: 'Login',
     description: 'Logge dich ein',
     icon: 'mdi-login',
-    visible: Visibility.Hidden,
+    visible: Visibility.Unauthenticated,
     meta: {
       path: '/login',
       name: 'Login',