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-tauri/src/lib.rs

@@ -0,0 +1,11 @@
+#[cfg_attr(mobile, tauri::mobile_entry_point)]
+pub fn run() {
+    let builder = tauri::Builder::default();
+
+    #[cfg(mobile)]
+    let builder = builder.plugin(tauri_plugin_barcode_scanner::init());
+
+    builder
+        .run(tauri::generate_context!())
+        .expect("error while running tauri application");
+}