Add frontend auth, dashboard & router guards

Introduce a complete frontend auth flow and protected dashboard.

- Add auth session module (GUI/src/services/authSession.ts) with fetchCurrentUser, login, logout, caching and structured errors.
- Add Dashboard page (GUI/src/routes/dashboard/Dashboard.vue) and a protected Dashboard route (meta.requiresAuth) at '/'.
- Move public landing page to /welcome and mark it Visibility.Unauthenticated; update 404 and Impressum links.
- Implement router guard (GUI/src/router/index.ts) to redirect unauthenticated users to Login and prevent logged-in users from accessing guest-only pages.
- Update routes layout (GUI/src/plugins/routesLayout.ts) to include authenticated/unauthenticated visibility and dashboard entry.
- Update Layout.vue to track current user, show username/menu, conditionally render sidebar items, add logout flow and error snackbar, and insert visual divider before auth-only items.
- Convert Login.vue into a working login form with loading state, error handling and redirect after success.
- Update codexInfo.md to document the new auth features and related UI/route changes.

GUI/src/plugins/routesLayout.ts

@@ -1,6 +1,7 @@
 import type { RouteRecordRaw } from 'vue-router'
 
 import Home from '@/routes/Home.vue'
+import Dashboard from '@/routes/dashboard/Dashboard.vue'
 import NotFound from '@/routes/404NotFound.vue'
 import Login from '@/routes/authentication/Login.vue'
 import Impressum from '@/routes/Impressum.vue'
@@ -38,17 +39,32 @@ export interface LayoutRoute {
  */
 export const routes: LayoutRoute[] = [
   {
-    path: '/',
+    path: '/welcome',
     name: 'Startseite',
     description: 'Self-hosted Datei-Workspace für Hoard',
     icon: 'mdi-home',
-    visible: Visibility.Public,
+    visible: Visibility.Unauthenticated,
     meta: {
       name: 'Home',
-      path: '/',
+      path: '/welcome',
       component: Home,
     },
   },
+  {
+    path: '/',
+    name: 'Dash',
+    description: 'Geschützter Bereich für dein Konto',
+    icon: 'mdi-view-dashboard-outline',
+    visible: Visibility.Authenticated,
+    meta: {
+      name: 'Dashboard',
+      path: '/',
+      component: Dashboard,
+      meta: {
+        requiresAuth: true,
+      },
+    },
+  },
   {
     path: '/login',
     name: 'Login',
@@ -59,6 +75,9 @@ export const routes: LayoutRoute[] = [
       path: '/login',
       name: 'Login',
       component: Login,
+      meta: {
+        guestOnly: true,
+      },
     },
   },
   {