chore: upgrade Angular to v21

ng update @angular/core@21 @angular/cli@21 (also bumped TypeScript to
5.9.3, required by v21), plus bump @fortawesome/angular-fontawesome to
4.0.0 (only major compatible with Angular 21 as peer dependency).

Mandatory core migrations for this version:
- src/main.ts: bootstrapApplication now gets an explicit
  provideZoneChangeDetection() provider, since the old implicit
  default bootstrap option was deprecated/removed. Preserves current
  zone-based change detection behavior, no functional change.
- slider-event: only template in the app using *ngFor - migrated
  automatically to the @for block-syntax (track card), CommonModule
  import dropped since it's no longer needed.

Build and test suite (7/7) pass.

Author: samuelsonbrito