Add game join UI and OnlineGame scaffold

Rename JoinMenu to GameJoinMenu and add a JoinGameObject model + OTP input for entering a 6-digit game code. Introduce a new JoinGameObject interface file. Update OnlineMode.vue to import and render GameJoinMenu and GameEndedMenu, add refs for joiningModel, game, gameField, currentSelectionIndex and gameEndedInformation, wire create/ join handlers (stubbed) and the main game view (Field and InfoField). Refactor LocalGame to OnlineGame, export it as default and add a stubbed drop method; adjust import path formatting. Several functions remain as stubs to be implemented in follow-up commits.

GUI/src/components/GameJoinMenu.vue

@@ -0,0 +1,35 @@
+<script setup lang="ts">
+import type JoinGameObject from '@/scripts/interfaces/JoinGameObject';
+
+defineEmits(['join']);
+
+let joiningModel = defineModel<JoinGameObject>('joinGameObject', {
+  required: true,
+});
+</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>
+      <v-otp-input
+        length="6"
+        v-model="joiningModel.gameCode"
+        type="number"
+        inputmode="numeric"
+        :error="joiningModel.failed"
+      ></v-otp-input>
+      <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>