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/web/vite.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from 'vite';
+import react from '@vitejs/plugin-react';
+import tailwindcss from '@tailwindcss/vite';
+
+export default defineConfig({
+  plugins: [tailwindcss(), react()],
+  server: {
+    port: 3000,
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8080',
+        changeOrigin: true,
+      },
+    },
+  },
+});