Integrate join codes, player management, and themes

This update introduces "join codes" for games to simplify game joining. Enhancements include player role and winner management for better organization. Additionally, theme preferences are now user-configurable and persisted server-side.

frontend/src/components/GamePickerCard.jsx

@@ -1,13 +1,9 @@
 // src/components/GamePickerCard.jsx
 import React from "react";
 import { styles } from "../styles/styles";
+import { stylesTokens } from "../styles/theme";
 
-export default function GamePickerCard({
-  games,
-  gameId,
-  setGameId,
-  onOpenHelp,
-}) {
+export default function GamePickerCard({ games, gameId, setGameId, joinCode, onOpenHelp }) {
   return (
     <div style={{ marginTop: 14 }}>
       <div style={styles.card}>
@@ -30,6 +26,19 @@ export default function GamePickerCard({
             Hilfe
           </button>
         </div>
+
+        {!!joinCode && (
+          <div
+            style={{
+              padding: "0 12px 12px",
+              fontSize: 12,
+              opacity: 0.85,
+              color: stylesTokens.textDim,
+            }}
+          >
+            Spiel-Code: <b style={{ color: stylesTokens.textGold }}>{joinCode}</b>
+          </div>
+        )}
       </div>
     </div>
   );