Add invoice template editor

frontend/src/layout/AppShell.tsx

@@ -5,6 +5,7 @@ const PAGE_TITLES: Record<string, string> = {
   "/home": "Startseite",
   "/samples/new": "Neuanlage einer Probe",
   "/portal": "MUH-Portal",
+  "/invoice-template": "Rechnungstemplate",
 };
 
 function resolvePageTitle(pathname: string) {
@@ -79,6 +80,9 @@ export default function AppShell() {
               <div className="nav-group">
                 <div className="nav-group__label">Verwaltung</div>
                 <div className="nav-subnav">
+                  <NavLink to="/invoice-template" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
+                    Rechnungstemplate
+                  </NavLink>
                   <NavLink to="/admin/landwirte" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
                     Landwirte
                   </NavLink>