fix: use npm install in Dockerfile, add static file serving

- Dockerfile uses npm install (no lockfile yet, kaniko builds in-cluster)
- Server serves client build from ./public in production
- SPA fallback for client-side routing

Dockerfile

@@ -1,16 +1,16 @@
 # Stage 1: Build client
 FROM node:20-alpine AS client-build
 WORKDIR /app/client
-COPY client/package.json client/package-lock.json* ./
-RUN npm ci
+COPY client/package.json ./
+RUN npm install
 COPY client/ ./
 RUN npm run build
 
 # Stage 2: Build server
 FROM node:20-alpine AS server-build
 WORKDIR /app/server
-COPY server/package.json server/package-lock.json* ./
-RUN npm ci
+COPY server/package.json ./
+RUN npm install
 COPY server/ ./
 RUN npm run build
 
@@ -25,8 +25,8 @@ RUN addgroup -g 1001 -S appgroup && \
     adduser -S appuser -u 1001 -G appgroup
 
 # Copy server production dependencies
-COPY server/package.json server/package-lock.json* ./
-RUN npm ci --omit=dev && npm cache clean --force
+COPY server/package.json ./
+RUN npm install --omit=dev && npm cache clean --force
 
 # Copy server build output
 COPY --from=server-build /app/server/dist ./dist