fix: pass NEXT_PUBLIC_* env vars as build args for Supabase client

Next.js inlines NEXT_PUBLIC_* vars at build time. They must be
available as ARGs during the Docker build, not just as runtime
environment variables.

frontend/Dockerfile

@@ -10,6 +10,10 @@ WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 ENV API_URL=http://backend:8080
+ARG NEXT_PUBLIC_SUPABASE_URL
+ARG NEXT_PUBLIC_SUPABASE_ANON_KEY
+ENV NEXT_PUBLIC_SUPABASE_URL=$NEXT_PUBLIC_SUPABASE_URL
+ENV NEXT_PUBLIC_SUPABASE_ANON_KEY=$NEXT_PUBLIC_SUPABASE_ANON_KEY
 RUN mkdir -p public
 RUN bun run build