build(portal-frontend): add Dockerfile and SPA nginx config

Multi-stage: node build with --base-href=/portal/, then nginx:alpine
serving the compiled dist with try_files SPA fallback for client-side
routes (e.g. a direct hit on /legacy/1001). Verified standalone: root
and a deep link both 200, base href correct.

portal-frontend/Dockerfile

@@ -0,0 +1,11 @@
+FROM node:22-alpine AS build
+WORKDIR /src
+COPY package.json package-lock.json ./
+RUN npm ci
+COPY . .
+RUN npx ng build --configuration production --base-href=/portal/
+
+FROM nginx:alpine AS runtime
+COPY --from=build /src/dist/portal-frontend/browser /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80

portal-frontend/nginx.conf

@@ -0,0 +1,8 @@
+server {
+  listen 80;
+  root /usr/share/nginx/html;
+
+  location / {
+    try_files $uri $uri/ /index.html;
+  }
+}