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/i18n/de.json

@@ -10,6 +10,8 @@
 		"description": "Fügen Sie jedes Urlaubskonto und seinen Gültigkeitszeitraum hinzu.",
 		"addVacationDay": "Urlaubskonto hinzufügen",
 		"calculate": "Plan berechnen",
+		"durationEstimate": "Ungefähre Wartezeit: {range} {unit}",
+		"durationUnits": { "seconds": "s", "minutes": "min" },
 		"remainingDaysWarning": "{count} nicht platziert:",
 		"remainingDaysManualPlace": "Bitte manuell platzieren.",
 		"deleteEntry": "Eintrag löschen",