feat: Phase 2 — auth, node/edge modals, API wiring

- Login page: full-screen dark, grid bg, JWT flow
- Auth store (Zustand) + API client (axios + interceptors)
- NodeModal: add/edit form (type, label, hostname, IP, check method)
- EdgeModal: link type picker + VLAN ID + label on connect
- App: auth gate, canvas load from API (fallback to demo), Ctrl+S save
- CanvasContainer: expose onConnect prop for edge modal flow

frontend/src/components/ui/label.tsx

@@ -0,0 +1,18 @@
+import * as React from "react"
+
+import { cn } from "@/lib/utils"
+
+function Label({ className, ...props }: React.ComponentProps<"label">) {
+  return (
+    <label
+      data-slot="label"
+      className={cn(
+        "flex items-center gap-2 text-sm leading-none font-medium select-none group-data-[disabled=true]:pointer-events-none group-data-[disabled=true]:opacity-50 peer-disabled:cursor-not-allowed peer-disabled:opacity-50",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+export { Label }