refactor(planning): remove the unused remaining-balance mirror

The summary and details already read the displayed plan's remaining
balances, so the writable mirror, its legacy conversion and the VacDay
classification/reason fields it alone populated are removed.

Refs #274

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/services/user-input-service.ts

@@ -161,7 +161,6 @@ export class UserInputService {
 	readonly vacationNumberSignal = signal<VacDay[]>(
 		this.getSavedValues() ?? this.getInitialValues(),
 	);
-	readonly remainingVacationDaysSignal = signal<VacDay[]>([]);
 	readonly planningOutcomeSignal = signal<PlanningOutcome | null>(null);
 	readonly planningSnapshotSignal = signal<PlanningSnapshot | null>(null);
 	readonly displayedPlanSignal = computed(() => {