Frontend: fix RxJS subscription leaks and add lazy loading

[pcastelo]

Problem

1. No subscription cleanup: Zero uses of ngOnDestroy, DestroyRef, or takeUntilDestroyed(). All components use bare .subscribe() without teardown. Repeated navigation accumulates handlers and causes memory leaks.

2. No lazy loading: All feature routes eagerly import components at the top of app.routes.ts. No loadComponent / loadChildren. Increases initial bundle size unnecessarily.

Acceptance criteria

Subscription management

• Add DestroyRef + takeUntilDestroyed() to all component subscriptions (Angular 16+ pattern)
• Or use async pipe in templates where possible (preferred)
• Verify no leaked subscriptions with a test

Lazy loading

• Convert feature routes to use loadComponent:

  { path: 'migrate', loadComponent: () => import('./features/migration/migration-wizard.component').then(m => m.MigrationWizardComponent) }

• Verify lazy chunks are generated in ng build output
• Measure bundle size reduction