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/user-input/user-input.html

@@ -30,11 +30,18 @@
 		class="calculate-button"
 		color="primary"
 		type="button"
+		[disabled]="!!calendarSettings.planningHorizonError()"
+		[attr.aria-describedby]="
+			calendarSettings.planningHorizonError() ? 'invalid-planning-horizon' : null
+		"
 		(click)="triggerComputation()"
 	>
 		<mat-icon aria-hidden="true">auto_awesome</mat-icon>
 		{{ tr().userInput.calculate }}
 	</button>
+	@if (calendarSettings.planningHorizonError()) {
+		<p id="invalid-planning-horizon" role="alert">{{ tr().settings.invalidHorizon }}</p>
+	}
 
 	@if (hasRemainingDays()) {
 		<mat-card class="warning-card">