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/components/WinnerCard.jsx

@@ -0,0 +1,33 @@
+import React from "react";
+import { styles } from "../styles/styles";
+import { stylesTokens } from "../styles/theme";
+
+export default function WinnerCard({ value, setValue, onSave }) {
+  return (
+    <div style={{ marginTop: 14 }}>
+      <div style={styles.card}>
+        <div style={styles.sectionHeader}>Sieger</div>
+
+        <div style={styles.cardBody}>
+          <input
+            value={value}
+            onChange={(e) => setValue(e.target.value)}
+            placeholder="Name des Siegers"
+            style={{ ...styles.input, flex: 1 }}
+            onKeyDown={(e) => {
+              if (e.key === "Enter") onSave();
+            }}
+          />
+
+          <button onClick={onSave} style={styles.primaryBtn} title="Speichern">
+            Speichern
+          </button>
+        </div>
+
+        <div style={{ padding: "0 12px 12px", fontSize: 12, color: stylesTokens.textDim }}>
+          Wird pro Spiel lokal gespeichert.
+        </div>
+      </div>
+    </div>
+  );
+}