fix: switch to single-stage build to eliminate multi-stage cache issues

fix: add ARG cache bust to force COPY --from=build-step to run fresh

Fix: Copy frontend build before project files to prevent overwrite

fix: replace npm ci with npm install to fix picomatch lock file mismatch

fix: default light mode + lock deps with npm ci in both Dockerfiles

build: prepare Dockerfile and FastAPI adjustments for Hugging Face Spaces deployment