fix(docker): Ensure public directory exists in web builder

The production stage was failing because it tried to copy the public
directory which doesn't exist in the source. Added mkdir -p to ensure
the directory exists (even if empty) before the production stage
tries to copy it.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/Dockerfile

@@ -47,6 +47,9 @@ ENV NEXT_PUBLIC_API_URL=${NEXT_PUBLIC_API_URL}
 # This ensures @mosaic/shared and @mosaic/ui are built first
 RUN pnpm turbo build --filter=@mosaic/web
 
+# Ensure public directory exists (may be empty)
+RUN mkdir -p ./apps/web/public
+
 # ======================
 # Production stage
 # ======================