feat(planner): implement cancellation for planning calculations

- Refactor leave-planner.worker.ts to support cancellation of ongoing calculations.
- Introduce `calculatePlanningAsyncInternal` for asynchronous planning calculations.
- Add cancellation handling in the worker message event.
- Update planning-date.ts with new utility functions for date manipulation.
- Extend translation-service.ts and translation files (en.json, de.json, es.json, fr.json) to include new strings related to planning horizon and cancellation.
- Create documentation for the cancellation feature in planner-cancellation.md.
- Add unit tests for cancellation functionality in leave-planner-cancellation.spec.ts and leave-planner.worker.spec.ts.

#57 #58

src/app/components/calendar-month/calendar-month.ts

@@ -79,7 +79,7 @@ export class CalendarMonth implements OnInit {
 
 	getDateKey(day: Date): string {
 		return [
-			day.getFullYear(),
+			String(day.getFullYear()).padStart(4, '0'),
 			String(day.getMonth() + 1).padStart(2, '0'),
 			String(day.getDate()).padStart(2, '0'),
 		].join('-');