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/stores/authStore.ts

@@ -0,0 +1,15 @@
+import { create } from 'zustand'
+
+interface AuthState {
+  token: string | null
+  isAuthenticated: boolean
+  login: (token: string) => void
+  logout: () => void
+}
+
+export const useAuthStore = create<AuthState>((set) => ({
+  token: null,
+  isAuthenticated: false,
+  login: (token) => set({ token, isAuthenticated: true }),
+  logout: () => set({ token: null, isAuthenticated: false }),
+}))