Activated flag strictNullInputTypes on Angular 21

[cdavalos7]

Motivation for features / changes

Enable strictNullInputTypes to strengthen Angular template type safety, catch nullable input bindings at build time, and prevent UI errors caused by loading or unavailable data. The change also modernizes synchronous NgRx selector bindings with Angular signals and reduces redundant reactive subscriptions.

Technical description of changes

• Enabled strictNullInputTypes: true in tsconfig.json.
• Migrated synchronous NgRx selector bindings from Observable + AsyncPipe to selectSignal or toSignal({requireSync: true}).
• Kept AsyncPipe for genuinely asynchronous streams and widened their receiving inputs when null is a valid value.
• Updated input types, template guards, and fallback values to handle null and undefined explicitly.
• Updated tests and NgRx mocks to provide synchronous selector state where required.
• Simplified three containers by replacing duplicate RxJS-to-Signal conversions with computed derived state.