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-year/calendar-year.html

@@ -14,14 +14,25 @@
 					></mat-spinner>
 					<div class="spinner-text">{{ loadingProgress() }}%</div>
 				</div>
-				<p role="status">{{ tr().loading }}</p>
+				<p role="status">{{ isCancelling() ? tr().planning.cancelling : tr().loading }}</p>
+				<button
+					mat-stroked-button
+					type="button"
+					class="cancel-calculation"
+					[disabled]="isCancelling()"
+					(click)="cancelCalculation()"
+				>
+					{{ tr().planning.cancel }}
+				</button>
 			</div>
 		</div>
 	}
 
 	<header class="results-header">
 		<div class="results-heading">
-			<h2 id="planning-results-title" tabindex="-1">{{ tr().planning.title }}</h2>
+			<h2 #resultsTitle id="planning-results-title" tabindex="-1">
+				{{ tr().planning.title }}
+			</h2>
 			@if (planningOutcome()?.kind === 'InvalidInput') {
 				<p class="planning-status error" role="alert">
 					<mat-icon aria-hidden="true">error</mat-icon>
@@ -42,7 +53,7 @@
 					<mat-icon aria-hidden="true">{{
 						hasPlannerError() ? 'error' : 'warning'
 					}}</mat-icon>
-					{{ hasPlannerError() ? tr().planning.error : tr().planning.incomplete }}
+					{{ outcomeLabel(planningOutcome()!) }}
 				</p>
 			} @else if (planningOutcome()?.kind === 'Plan') {
 				<p class="planning-status success" role="status">