Sprint 8: modernes Design-System für die React-PWA

Gemeinsames Stylesheet (Tokens, Karten, Buttons, Badges, Formulare) plus
AppShell (Kopfzeile mit Navigation/Logout) statt Inline-Styles je Screen.
Kein UI-Framework als neue Abhängigkeit - schlankes eigenes CSS, damit
Build/Deploy-Weg unverändert bleibt.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L85hmKbvX7Cqkq47KnQhFt

frontend/src/App.tsx

@@ -2,6 +2,7 @@ import type { ReactNode } from "react";
 import { Navigate, Route, Routes } from "react-router-dom";
 
 import { AuthProvider, useAuth } from "./auth/AuthContext";
+import { AppShell } from "./components/AppShell";
 import { AdminPage } from "./pages/AdminPage";
 import { KontrollPage } from "./pages/KontrollPage";
 import { LoginPage } from "./pages/LoginPage";
@@ -12,7 +13,7 @@ function GeschuetzteRoute({ children }: { children: ReactNode }) {
   if (!eingeloggt) {
     return <Navigate to="/login" replace />;
   }
-  return <>{children}</>;
+  return <AppShell>{children}</AppShell>;
 }
 
 function AdminRoute({ children }: { children: ReactNode }) {
@@ -23,7 +24,7 @@ function AdminRoute({ children }: { children: ReactNode }) {
   if (!istAdmin) {
     return <Navigate to="/objekte" replace />;
   }
-  return <>{children}</>;
+  return <AppShell>{children}</AppShell>;
 }
 
 export default function App() {