Integrate Vuetify layout and routing

Add a Vuetify-powered application shell (Layout.vue) replacing the previous App.vue, including app bar, navigation drawer, theme toggle, footer and localStorage persistence for theme/drawer. Introduce a routesLayout plugin with a Visibility enum and centralized LayoutRoute definitions; add route components (Home, Impressum, Login, 404NotFound) and update the router to build routes from the new layout definitions. Register Vuetify in main.ts and add dependencies (vuetify, @fontsource/roboto, @mdi/font) in package.json; update tsconfig.app.json to include .ts files. Package-lock.json updated accordingly.

GUI/src/routes/404NotFound.vue

@@ -0,0 +1,30 @@
+<script setup lang="ts"></script>
+
+<template>
+  <v-container fluid class="fill-height d-flex flex-column justify-center align-center">
+    <h1 class="error-title text-h1 font-weight-bold">404</h1>
+    <p class="error-message text-h5">Seite nicht gefunden</p>
+
+    <router-link to="/" class="text-primary text-decoration-none font-weight-medium mt-5">
+      Zurueck zur Startseite
+    </router-link>
+  </v-container>
+</template>
+
+<style scoped>
+.error-title {
+  font-size: 3rem;
+  font-weight: bold;
+  margin-bottom: 1rem;
+  letter-spacing: 2px;
+}
+
+.error-message {
+  font-size: 1.25rem;
+  opacity: 0.85;
+}
+
+a:hover {
+  text-decoration: underline !important;
+}
+</style>