Display user first name in dashboard greeting

- Extract firstName/lastName from auth me() endpoint (already provided by backend)
- Show first name in greeting, fallback to username if not set
- Remove hardcoded 'Sabina & Malik' greeting

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

omsorgapp/src/components/Header.jsx

@@ -3,13 +3,14 @@ import { useAuth } from "../app/AuthContext";
 
 export default function Header() {
   const { user, logout } = useAuth();
+  const displayName = user?.firstName || user?.username || "Benutzer";
 
   return (
     <header className="header">
 
       <div className="header-left">
 
-        <h1>Guten Morgen Sabina & Malik ☀️</h1>
+        <h1>Guten Morgen {displayName} ☀️</h1>
 
         <p>
           Schön, dass ihr heute wieder da seid.