fix(Docker): ship prebuilt frontend/dist, drop npm from image (removes 'vite not found')

The Vue SPA is now built locally and its frontend/dist/ committed (force-added).
Dockerfile is a single-stage runtime: copy prebuilt dist + python:3.11 + Flask.
No node/npm inside the image -> deterministic, no npm/esbuild postinstall flakiness.
To update UI: edit src, npm run build, commit frontend/dist/.

Dockerfile

@@ -1,27 +1,29 @@
-# Sales Trainer — single-container build (Vue frontend built + served by Flask)
-FROM python:3.11-slim AS backend
+# Sales Trainer — single-container runtime (Python 3.11 + Flask)
+#
+# The Vue frontend is BUILT LOCALLY (`cd frontend && npm run build`) and the
+# resulting `frontend/dist/` is committed to the repo. This image does NOT run
+# npm/node at all — it just copies the prebuilt SPA and serves it via Flask.
+# This eliminates `vite: not found` / npm postinstall flakiness from the build.
+#
+# To update the UI: edit frontend/src, run `npm run build` locally, commit dist/.
 
-# Node 20 for building the Vue frontend
+FROM python:3.11-slim
+
+# Runtime deps (none required beyond Python + pip packages; curl for healthcheck)
 RUN apt-get update \
   && apt-get install -y --no-install-recommends curl ca-certificates \
-  && curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
-  && apt-get install -y --no-install-recommends nodejs \
   && rm -rf /var/lib/apt/lists/*
 
-ENV NODE_ENV=production
 WORKDIR /app
 
-# 1) Build frontend
-COPY frontend/package.json frontend/package-lock.json* ./frontend/
-RUN cd frontend && npm install
-COPY frontend/ ./frontend/
-RUN cd frontend && npm run build
+# 1) Copy prebuilt frontend SPA (built locally, committed to repo)
+COPY frontend/dist/ ./frontend/dist/
 
 # 2) Install backend deps
 COPY backend/requirements.txt ./backend/
 RUN pip install --no-cache-dir -r backend/requirements.txt
 
-# 3) Copy backend source
+# 3) Copy backend source (and stub frontend dir so static serving finds dist)
 COPY backend/ ./backend/
 
 # Run