Add Ping frontend

React 19 + TypeScript client built with Vite. Zustand for local state,
react-oidc-context for the login flow, a SignalR connection for chat and
presence events, and LiveKit's React components for voice/video calls.

frontend/src/store/toastStore.ts

@@ -0,0 +1,27 @@
+import { create } from 'zustand'
+
+export type ToastTone = 'error' | 'info'
+
+export interface Toast {
+  id: number
+  message: string
+  tone: ToastTone
+}
+
+let nextId = 1
+
+interface ToastState {
+  toasts: Toast[]
+  push: (message: string, tone?: ToastTone) => void
+  dismiss: (id: number) => void
+}
+
+export const useToastStore = create<ToastState>((set) => ({
+  toasts: [],
+  push: (message, tone = 'info') => {
+    const id = nextId++
+    set((s) => ({ toasts: [...s.toasts, { id, message, tone }] }))
+    setTimeout(() => set((s) => ({ toasts: s.toasts.filter((t) => t.id !== id) })), 5000)
+  },
+  dismiss: (id) => set((s) => ({ toasts: s.toasts.filter((t) => t.id !== id) })),
+}))