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/layout/AppShell.tsx

@@ -8,6 +8,7 @@ const PAGE_TITLES: Record<string, string> = {
   "/samples/new": "Neuanlage einer Probe",
   "/portal": "MUH-Portal",
   "/report-template": "Bericht",
+  "/admin/stammdaten": "Meine Stammdaten",
   "/admin/preistabelle": "Preistabelle",
   "/admin/rechnung/verwalten": "Rechnungsverwaltung",
   "/admin/rechnung/template": "Rechnungsvorlage",
@@ -65,6 +66,13 @@ export default function AppShell() {
                 </div>
               </div>
 
+              <NavLink
+                to="/admin/stammdaten"
+                className={({ isActive }) => `nav-link ${isActive ? "is-active" : ""}`}
+              >
+                Stammdaten
+              </NavLink>
+
               <NavLink
                 to="/admin/preistabelle"
                 className={({ isActive }) => `nav-link ${isActive ? "is-active" : ""}`}