feat: fase 3 - frontend next.js setup

- Next.js 15 + TypeScript + Bootstrap 5 + FontAwesome
- Layout replica SAM.Master (sidebar collapsable + header)
- Sidebar, Header, LoadingSpinner, ModalConfirmacion components
- Login page with RUT validation
- Dashboard page with summary cards
- AuthContext + JWT auth with cookies
- api.ts centralized HTTP client
- useInactividad hook (30min timeout)
- validarRut.ts + utils.ts migrated
- Original CSS + images copied from project
- Dockerfile multi-stage build

frontend/Dockerfile

@@ -0,0 +1,14 @@
+FROM node:20-alpine AS build
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+FROM node:20-alpine AS runtime
+WORKDIR /app
+COPY --from=build /app/.next/standalone ./
+COPY --from=build /app/public ./public
+EXPOSE 3000
+ENV NODE_ENV=production
+CMD ["node", "server.js"]