feat: complete fases 3,4,5 - frontend, containers, e2e tests

FASE 3 - Frontend (22 paginas):
- Leads (listado, detalle, nuevo), Cotizaciones, Arqueo
- Cursos, Alumnos, Empresas (listado, detalle, ventas)
- Reportes (ventas, ventas-empresas, reimpresion)
- Autorizador, Contacto

FASE 4 - Contenedores:
- docker-compose.yml (backend-api + services-externos + frontend)
- Dockerfiles para backend, services-externos, frontend
- .env.example con secretos

FASE 5 - Pruebas E2E:
- Playwright config + 3 spec files (login, leads, dashboard)
- 7 escenarios: login, error, sidebar, crear lead,
  dashboard cards, timeout, reportes

services-externos/src/ServicesExternos.API/Dockerfile

@@ -0,0 +1,11 @@
+FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+WORKDIR /src
+COPY . .
+RUN dotnet restore ServicesExternos.slnx
+RUN dotnet publish src/ServicesExternos.API/ServicesExternos.API.csproj -c Release -o /app
+
+FROM mcr.microsoft.com/dotnet/aspnet:10.0
+WORKDIR /app
+COPY --from=build /app .
+EXPOSE 8080
+ENTRYPOINT ["dotnet", "ServicesExternos.API.dll"]