Add theme customization and winner management features

Introduced a theme selection feature, allowing users to customize the application's appearance, with themes stored per user. Added functionality to manage and store the game's winner locally. These changes improve user experience and personalization.

frontend/src/utils/winnerStorage.js

@@ -0,0 +1,20 @@
+function winnerKey(gameId) {
+    return `winner:${gameId}`;
+  }
+
+  export function getWinner(gameId) {
+    if (!gameId) return "";
+    try {
+      return localStorage.getItem(winnerKey(gameId)) || "";
+    } catch {
+      return "";
+    }
+  }
+
+  export function setWinner(gameId, name) {
+    if (!gameId) return;
+    try {
+      localStorage.setItem(winnerKey(gameId), name || "");
+    } catch {}
+  }
+