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-settings/user-settings.html

@@ -2,6 +2,53 @@
 <mat-dialog-content>
 	<p>{{ tr().settings.description }}</p>
 	<div class="settings-controls">
+		<fieldset class="planning-dates">
+			<legend>{{ tr().settings.planningHorizon }}</legend>
+			<p id="planning-dates-help">{{ tr().settings.planningDatesHelp }}</p>
+			<mat-form-field appearance="outline">
+				<mat-label>{{ tr().settings.planningStart }}</mat-label>
+				<input
+					matInput
+					type="date"
+					name="planningStart"
+					required
+					#planningStart
+					[value]="calendarSettings.planningDates().start"
+					[attr.aria-invalid]="!!calendarSettings.planningHorizonError()"
+					aria-describedby="planning-dates-help planning-dates-error"
+					(input)="
+						calendarSettings.setPlanningDates(
+							planningStart.value,
+							lastPlanningDate.value
+						)
+					"
+				/>
+			</mat-form-field>
+			<mat-form-field appearance="outline">
+				<mat-label>{{ tr().settings.lastPlanningDate }}</mat-label>
+				<input
+					matInput
+					type="date"
+					name="lastPlanningDate"
+					required
+					#lastPlanningDate
+					[value]="calendarSettings.planningDates().lastPlanningDate"
+					[attr.aria-invalid]="!!calendarSettings.planningHorizonError()"
+					aria-describedby="planning-dates-help planning-dates-error"
+					(input)="
+						calendarSettings.setPlanningDates(
+							planningStart.value,
+							lastPlanningDate.value
+						)
+					"
+				/>
+			</mat-form-field>
+			@if (calendarSettings.planningHorizonError(); as error) {
+				<p id="planning-dates-error" class="validation-error" role="alert">
+					{{ tr().settings[error] }}
+				</p>
+			}
+		</fieldset>
 		<mat-form-field appearance="outline">
 			<mat-label>{{ tr().settings.selectTheme }}</mat-label>
 			<mat-select