Support player names and online join flow

Move SignalR group join into GameManager and add support for player names and improved online join flow.

- Moved Groups.AddToGroupAsync call out of GameHubSocket into GameManager (hubContext.Groups.AddToGroupAsync) so group membership is handled when adding players to a game.
- GUI: added player name input to GameJoinMenu and updated JoinGameObject to include playerName and use string gameCode.
- OnlineMode & LocalMode: constructors updated to instantiate games without passing settings; removed WaitingForOpponent state and cleaned up event bindings.
- OnlineGame: rewrote to handle onGameCreated/onGameJoined/onGameStarted/onFieldUpdated/onGameEnded/onError, added joinGame validation (expects 6-digit code), join flow (connect + join), state updates, drop forwarding to player.drop(gameId, index) and place description updates.
- LocalGame: constructor signature simplified to no-arg.

These changes centralize group management, add player identification, validate join codes, and improve game state/event handling for online play.

GUI/src/components/GameJoinMenu.vue

@@ -17,6 +17,11 @@ let joiningModel = defineModel<JoinGameObject>('joinGameObject', {
         dein Freund gegeben hat.
       </h3>
       <v-divider class="mb-5 mt-5"></v-divider>
+      <v-text-field
+            v-model="joiningModel.playerName"
+            label="Name vom Spieler 1"
+            required
+          ></v-text-field>
       <v-otp-input
         length="6"
         v-model="joiningModel.gameCode"