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

Dockerfile

@@ -1,16 +1,12 @@
-# Stage 1: Build the React Frontend
-FROM node:20-alpine AS build-step
-
-WORKDIR /app/frontend
-COPY frontend/package*.json ./
-RUN npm install --ignore-scripts
-
-COPY frontend/ ./
-RUN npm run build
-
-# Stage 2: Serve with Python FastAPI
+# Single-stage build: Node + Python in one image (avoids multi-stage cache issues)
 FROM python:3.10-slim
 
+# Install Node.js 20 as root
+RUN apt-get update && apt-get install -y curl && \
+    curl -fsSL https://deb.nodesource.com/setup_20.x | bash - && \
+    apt-get install -y nodejs && \
+    rm -rf /var/lib/apt/lists/*
+
 # Set up a new user named "user" with user ID 1000 (Required by Hugging Face Spaces)
 RUN useradd -m -u 1000 user
 USER user
@@ -18,15 +14,19 @@ ENV PATH="/home/user/.local/bin:$PATH"
 
 WORKDIR /home/user/app
 
-# Copy requirement files and install Python dependencies
+# Install Python dependencies (cached separately)
 COPY --chown=user:user requirements.txt ./
 RUN pip install --no-cache-dir --upgrade pip
 RUN pip install --no-cache-dir -r requirements.txt
 
-# Cache bust for frontend copy: v3
-ARG FRONTEND_COPY_BUST=3
-RUN mkdir -p /home/user/app/frontend/dist
-COPY --from=build-step --chown=user:user /app/frontend/dist/ /home/user/app/frontend/dist/
+# Build React frontend (in same stage, no cross-stage cache issue)
+COPY --chown=user:user frontend/package*.json ./frontend/
+RUN cd frontend && npm install --ignore-scripts
+
+COPY --chown=user:user frontend/ ./frontend/
+RUN cd frontend && npm run build
+
+# Copy remaining project files (does NOT overwrite frontend/dist/)
 COPY --chown=user:user . .
 
 EXPOSE 7860