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/vite-env.d.ts

@@ -0,0 +1,13 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_API_BASE_URL: string
+  readonly VITE_AUTH_AUTHORITY: string
+  readonly VITE_AUTH_CLIENT_ID: string
+  readonly VITE_AUTH_REDIRECT_URI: string
+  readonly VITE_LIVEKIT_URL: string
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv
+}