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

flake.nix

@@ -0,0 +1,81 @@
+{
+  description = "Effigenix ERP – Dev Environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { nixpkgs, flake-utils, rust-overlay, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs { inherit system overlays; };
+
+        rustToolchain = pkgs.rust-bin.stable.latest.default.override {
+          extensions = [ "rust-src" "rust-analyzer" ];
+          targets = [
+            "aarch64-linux-android"
+            "armv7-linux-androideabi"
+            "i686-linux-android"
+            "x86_64-linux-android"
+          ];
+        };
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          name = "effigenix";
+
+          buildInputs = with pkgs; [
+            # ─── Frontend ───────────────────────────
+            nodejs_22
+            nodePackages.pnpm
+
+            # ─── Rust / Tauri ───────────────────────
+            rustToolchain
+            cargo-tauri
+
+            # Tauri v2 system deps (Linux/GTK)
+            pkg-config
+            openssl
+            gtk3
+            webkitgtk_4_1
+            libsoup_3
+            glib
+            gdk-pixbuf
+            cairo
+            pango
+            harfbuzz
+            atk
+
+            # ─── Tools ─────────────────────────────
+            just
+            jq
+            curl
+            postgresql
+            rainfrog
+          ];
+
+          # Tauri braucht diese zur Laufzeit
+          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath (with pkgs; [
+            gtk3
+            webkitgtk_4_1
+            libsoup_3
+            openssl
+          ]);
+
+          shellHook = ''
+            echo "Node    $(node --version)"
+            echo "pnpm    $(pnpm --version)"
+            echo "rustc   $(rustc --version)"
+            echo "cargo   $(cargo --version)"
+            echo ""
+            echo "just --list  für alle Befehle"
+          '';
+        };
+      });
+}