fix(frontend): pass VITE_BE_API_URL and VITE_BE_WS_URL as Docker build args

Vite embeds env vars at build time, not runtime. The frontend code uses
VITE_BE_API_URL/VITE_BE_WS_URL but the workflow was setting wrong names
(VITE_API_URL/VITE_WS_URL). Now Dockerfile accepts build args and
workflow passes production URLs during docker build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/docker/Dockerfile.frontend

@@ -1,5 +1,8 @@
 FROM node:22-alpine
 
+ARG VITE_BE_API_URL
+ARG VITE_BE_WS_URL
+
 WORKDIR /app
 
 # Install pnpm
@@ -16,10 +19,10 @@ COPY services/frontend ./services/frontend
 # Install dependencies
 RUN pnpm install --frozen-lockfile
 
-# Build frontend
-RUN pnpm --filter './services/frontend' run build
+# Build frontend (env vars injected at build time)
+RUN VITE_BE_API_URL=${VITE_BE_API_URL} VITE_BE_WS_URL=${VITE_BE_WS_URL} pnpm --filter './services/frontend' run build
 
-# Expose port (served by backend)
+# Expose port
 EXPOSE 3000
 
 # Start frontend (production preview)