Add display_name support for users in backend and frontend

This commit introduces the `display_name` field to the user model. It updates database migrations, API endpoints, and the admin panel to handle this field. Additionally, the `display_name` is now shown in the TopBar and WinnerBadge components, improving user experience.

frontend/src/components/WinnerBadge.jsx

@@ -1,8 +1,26 @@
 import React from "react";
 import { stylesTokens } from "../styles/theme";
 
-export default function WinnerBadge({ winnerEmail }) {
-  if (!winnerEmail) return null;
+/**
+ * Props:
+ * - winner: { display_name?: string, email?: string } | null
+ *   (oder als Fallback:)
+ * - winnerEmail: string | null
+ */
+export default function WinnerBadge({ winner, winnerEmail }) {
+  const name =
+    (winner?.display_name || "").trim() ||
+    (winner?.email || "").trim() ||
+    (winnerEmail || "").trim();
+
+  if (!name) return null;
+
+  // Optional: wenn display_name vorhanden ist, Email klein anzeigen
+  const showEmail =
+    winner &&
+    (winner?.email || "").trim() &&
+    (winner?.display_name || "").trim() &&
+    winner.email.trim().toLowerCase() !== winner.display_name.trim().toLowerCase();
 
   return (
     <div
@@ -22,9 +40,18 @@ export default function WinnerBadge({ winnerEmail }) {
     >
       <div style={{ display: "flex", alignItems: "center", gap: 10 }}>
         <div style={{ fontSize: 18 }}>🏆</div>
-        <div style={{ color: stylesTokens.textMain, fontWeight: 900 }}>
-          Sieger:
-          <span style={{ color: stylesTokens.textGold }}>{" "}{winnerEmail}</span>
+
+        <div style={{ display: "grid", gap: 2 }}>
+          <div style={{ color: stylesTokens.textMain, fontWeight: 900 }}>
+            Sieger:
+            <span style={{ color: stylesTokens.textGold }}>{" "}{name}</span>
+          </div>
+
+          {showEmail && (
+            <div style={{ fontSize: 12, opacity: 0.8, color: stylesTokens.textDim }}>
+              {winner.email}
+            </div>
+          )}
         </div>
       </div>