feat: Add invoice management menu and template editor for admin

- Add 'Rechnung' menu with sub-items 'Verwalten' and 'Template' in admin sidebar
- Create InvoiceTemplatePage with drag-and-drop editor for invoice templates
  - Includes invoice-specific elements (header, customer data, issuer info,
    invoice items, totals, payment terms, bank details)
  - Supports PDF preview and download
  - API integration for saving/loading templates (/admin/invoice-template)
- Create InvoiceManagementPage as placeholder for invoice overview
- Add routes for /admin/rechnung/verwalten and /admin/rechnung/template
- Update page titles in AppShell for new routes

frontend/src/layout/AppShell.tsx

@@ -7,6 +7,8 @@ const PAGE_TITLES: Record<string, string> = {
   "/samples/new": "Neuanlage einer Probe",
   "/portal": "MUH-Portal",
   "/report-template": "Bericht",
+  "/admin/rechnung/verwalten": "Rechnungsverwaltung",
+  "/admin/rechnung/template": "Rechnungsvorlage",
 };
 
 function resolvePageTitle(pathname: string, isAdmin: boolean) {
@@ -58,6 +60,18 @@ export default function AppShell() {
                   </NavLink>
                 </div>
               </div>
+
+              <div className="nav-group">
+                <div className="nav-group__label">Rechnung</div>
+                <div className="nav-subnav">
+                  <NavLink to="/admin/rechnung/verwalten" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
+                    Verwalten
+                  </NavLink>
+                  <NavLink to="/admin/rechnung/template" className={({ isActive }) => `nav-sublink ${isActive ? "is-active" : ""}`}>
+                    Template
+                  </NavLink>
+                </div>
+              </div>
             </>
           ) : (
             <>