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/services/translation-service.ts

@@ -57,6 +57,13 @@ export interface AppTranslations {
 			dark: string;
 		};
 		systemAppearance: string;
+		planningHorizon: string;
+		planningStart: string;
+		lastPlanningDate: string;
+		planningDatesHelp: string;
+		invalidDate: string;
+		horizonOrder: string;
+		invalidHorizon: string;
 	};
 	rules: {
 		title: string;
@@ -191,6 +198,10 @@ export interface AppTranslations {
 		invalidInput: string;
 		conflict: string;
 		incomplete: string;
+		cancel: string;
+		cancelling: string;
+		cancelled: string;
+		cancelledPartial: string;
 		error: string;
 		complete: string;
 		whyPlacement: string;