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/Dockerfile

@@ -0,0 +1,28 @@
+# Build context: frontend/  (docker compose -f deploy/compose.yml --profile apps build)
+FROM node:24-alpine AS build
+WORKDIR /src
+
+COPY package.json package-lock.json ./
+RUN npm ci
+
+COPY . .
+
+# Vite inlines VITE_* variables at build time, so they have to be build args, not
+# runtime environment variables — see deploy/compose.yml's web service.
+ARG VITE_API_BASE_URL
+ARG VITE_AUTH_AUTHORITY
+ARG VITE_AUTH_CLIENT_ID
+ARG VITE_AUTH_REDIRECT_URI
+ARG VITE_LIVEKIT_URL
+ENV VITE_API_BASE_URL=$VITE_API_BASE_URL \
+    VITE_AUTH_AUTHORITY=$VITE_AUTH_AUTHORITY \
+    VITE_AUTH_CLIENT_ID=$VITE_AUTH_CLIENT_ID \
+    VITE_AUTH_REDIRECT_URI=$VITE_AUTH_REDIRECT_URI \
+    VITE_LIVEKIT_URL=$VITE_LIVEKIT_URL
+
+RUN npm run build
+
+FROM nginx:alpine AS runtime
+COPY --from=build /src/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 3100