CUB-66: scaffold Angular frontend and add Dockerfile with nginx

- Scaffolded Angular 21 app in frontend/ (standalone, routing, scss)
- Multi-stage Dockerfile: node:22-alpine build → nginx:alpine serve
- nginx.conf with SPA routing fallback, API proxy, gzip, asset caching
- .dockerignore excludes node_modules, dist, .angular, spec files
- docker build → PASS, container serves UI on port 80 (HTTP 200)
- Final image: 92.9MB (nginx:alpine)

frontend/src/app/app.ts

@@ -0,0 +1,12 @@
+import { Component, signal } from '@angular/core';
+import { RouterOutlet } from '@angular/router';
+
+@Component({
+  selector: 'app-root',
+  imports: [RouterOutlet],
+  templateUrl: './app.html',
+  styleUrl: './app.scss'
+})
+export class App {
+  protected readonly title = signal('frontend');
+}