Add JoinMenu and join error handling

Add a new JoinMenu.vue component to provide a UI for joining existing games (emits a "join" event and offers Cancel/Join buttons). Improve UX by adding spacing to GameCreationMenu.vue header. Update OnlineMode.vue to import GameCreationMenu. In the API, send a client-side error message when adding a player fails (GameManager.cs now notifies the player's connection with "Spiel Existiert nicht!" before returning null) so users receive immediate feedback when a join attempt is invalid.

GUI/src/components/GameCreationMenu.vue

@@ -17,7 +17,7 @@ function gameFieldPreset(x: number, y: number) {
 <template>
   <v-container class="d-flex align-center justify-center fill-height">
     <v-sheet width="100%" class="text-centered pa-2 w-75" rounded>
-      <h1 class="text-center">Spiel Erstellen</h1>
+      <h1 class="text-center mb-4">Spiel Erstellen</h1>
       <v-divider class="mb-4"></v-divider>
       <v-row>
         <v-col cols="12" md="6">

GUI/src/components/JoinMenu.vue

@@ -0,0 +1,21 @@
+<script setup lang="ts">
+defineEmits(["join"]);
+</script>
+
+<template>
+  <v-container class="d-flex align-center justify-center fill-height">
+    <v-sheet class="text-centered pa-2 w-50" rounded>
+      <h1 class="text-center mb-5">Spiel Beitreten</h1>
+      <h3 class="text-center">
+        Hier kannst du einem bestehenden Spiel beitreten, indem du den Spiel Code eingibst, die dir dein Freund gegeben hat.
+      </h3>
+      <v-divider class="mb-5 mt-5"></v-divider>
+      <div class="d-flex align-center justify-space-evenly ma-4 w-100">
+        <v-btn color="red" @click="$router.push('/')" rounded="xl"> Abbrechen </v-btn>
+        <v-btn color="primary" @click="$emit('join')" rounded="xl"> Beitreten </v-btn>
+      </div>
+    </v-sheet>
+  </v-container>
+</template>
+
+<style scoped></style>

GUI/src/routes/OnlineMode.vue

@@ -1,5 +1,6 @@
 <script setup lang="ts">
 import CreateOrJoinMenu from '@/components/CreateOrJoinMenu.vue';
+import GameCreationMenu from '@/components/GameCreationMenu.vue';
 import type { GameSettings } from '@/scripts/interfaces/GameSettings';
 import { ref } from 'vue';