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">