feat: Add Stammdaten page for admin to manage own profile

- Add AdminProfilePage component for admin to edit own data
- Add 'Stammdaten' menu item in admin navigation
- Add route /admin/stammdaten for the new page
- Use existing POST /api/portal/users endpoint to save changes
- Update session context after successful save

frontend/src/App.tsx

@@ -18,6 +18,7 @@ import ReportTemplatePage from "./pages/ReportTemplatePage";
 import InvoiceTemplatePage from "./pages/InvoiceTemplatePage";
 import InvoiceManagementPage from "./pages/InvoiceManagementPage";
 import PricingPage from "./pages/PricingPage";
+import AdminProfilePage from "./pages/AdminProfilePage";
 
 function ProtectedRoutes() {
   const { user, ready } = useSession();
@@ -48,6 +49,7 @@ function ProtectedRoutes() {
         <Route path="/admin/medikamente" element={<AdministrationPage />} />
         <Route path="/admin/erreger" element={<AdministrationPage />} />
         <Route path="/admin/antibiogramm" element={<AdministrationPage />} />
+        <Route path="/admin/stammdaten" element={<AdminProfilePage />} />
         <Route path="/admin/preistabelle" element={<PricingPage />} />
         <Route path="/admin/rechnung/verwalten" element={<InvoiceManagementPage />} />
         <Route path="/admin/rechnung/template" element={<InvoiceTemplatePage />} />