Store key_prefix for readable key display instead of masked hash

The last-4 of the SHA-256 hash was meaningless for identification.
Now storing the first 12 chars of the plaintext key as key_prefix,
displayed as 'sk-aBcDeFgH••••••••' — consistent with what the user
sees at creation time and how GitHub/OpenAI handle it.

frontend/src/main.jsx

@@ -3,7 +3,7 @@ import ReactDOM from 'react-dom/client';
 import axios from 'axios';
 import './styles.css';
 
-const maskKey = (key) => `••••••••${key.slice(-4)}`;
+const displayKey = (prefix) => prefix ? `${prefix}••••••••` : '••••••••••••';
 
 function authHeaders(token) {
   return { Authorization: `Bearer ${token}` };
@@ -298,7 +298,7 @@ function App() {
               <tr key={key.id}>
                 <td>{key.id}</td>
                 <td>{key.name}</td>
-                <td>{maskKey(key.key)}</td>
+                <td>{displayKey(key.key_prefix)}</td>
                 <td>{key.is_active ? 'Aktiv' : 'Inaktiv'}</td>
                 <td>{key.expires_at ? new Date(key.expires_at).toLocaleDateString('de-DE', { timeZone: 'Europe/Berlin' }) : '∞'}</td>
                 <td>{key.daily_tokens ?? '∞'}</td>