feat(frontend): Tauri Apps, Shared UI Library und Nix Flake

- packages/ui: Shared React Component Library (Button, Card, Badge, Input)
  mit Tailwind v4 @theme Design Tokens (oklch)
- apps/web: ERP Web-UI (Vite + React + Tailwind v4, API-Proxy :8080)
- apps/scanner: Tauri v2 Mobile App mit Barcode-Scanner Plugin
  (cfg(mobile) für Desktop-Kompatibilität)
- flake.nix: Nix Flake mit rust-overlay, Tauri System-Deps (GTK,
  WebKitGTK, libsoup, OpenSSL), ersetzt shell.nix
- justfile: Dev-Befehle für alle Projekte (backend, cli, web, scanner)
- frontend/CLAUDE.md: Agent Guide mit Base UI Docs Referenz

frontend/apps/scanner/src/App.tsx

@@ -0,0 +1,29 @@
+import { Button, Card } from '@effigenix/ui';
+
+export function App() {
+  return (
+    <div className="min-h-screen p-6">
+      <div className="mx-auto max-w-md space-y-6">
+        <h1 className="text-2xl font-bold text-brand-700">Effigenix Scanner</h1>
+
+        <Card>
+          <h2 className="mb-4 text-lg font-semibold">Chargen-Scanner</h2>
+          <p className="mb-4 text-sm text-warm-600">
+            QR-Code oder Barcode scannen, um Chargen-Details anzuzeigen.
+          </p>
+          <Button
+            variant="primary"
+            size="lg"
+            className="w-full"
+            onClick={() => {
+              // TODO: Barcode-Scanner Integration
+              console.log('Scan gestartet');
+            }}
+          >
+            Scan starten
+          </Button>
+        </Card>
+      </div>
+    </div>
+  );
+}

frontend/apps/scanner/src/index.css

@@ -0,0 +1,4 @@
+@import 'tailwindcss';
+@import '@effigenix/ui/theme.css';
+
+@source '../../packages/ui/src/**/*.{ts,tsx}';

frontend/apps/scanner/src/main.tsx

@@ -0,0 +1,10 @@
+import { StrictMode } from 'react';
+import { createRoot } from 'react-dom/client';
+import { App } from './App';
+import './index.css';
+
+createRoot(document.getElementById('root')!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>
+);

frontend/apps/scanner/src/vite-env.d.ts

@@ -0,0 +1 @@
+/// <reference types="vite/client" />