feat: add PlanningDurationService for estimating planning duration

- Implemented PlanningDurationService to provide an indicative wait time before calculation based on user input and calendar settings.
- Introduced duration estimation logic that considers horizon dates, balances, and rules without triggering a calculation.
- Enhanced user input component to display estimated duration using the new service.
- Updated translation files to include new duration estimate strings in multiple languages.
- Added tests for PlanningDurationService to ensure accurate duration estimation and calibration.
- Refactored leave planner service to remove hardcoded deadlines and improve worker responsiveness.

src/app/services/translation-service.ts

@@ -14,6 +14,8 @@ export interface AppTranslations {
 		description: string;
 		addVacationDay: string;
 		calculate: string;
+		durationEstimate: string;
+		durationUnits: { seconds: string; minutes: string };
 		remainingDaysWarning: string;
 		remainingDaysManualPlace: string;
 		deleteEntry: string;