refactor(calendar): drop the unused projection horizon

No caller read CalendarProjection.horizon, and with fixed months it no
longer described the listed months.

Refs #276

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/components/calendar-year/calendar-year-planning.spec.ts

@@ -142,11 +142,7 @@ function projectionFor(
 		months.push({ monthIndex: cursor.getMonth(), year: cursor.getFullYear() });
 		cursor.setMonth(cursor.getMonth() + 1);
 	}
-	return {
+	return { months, selections: selectedDatesForPlan(value, includeHoliday) };
-		horizon,
-		months,
-		selections: selectedDatesForPlan(value, includeHoliday),
-	};
 }
 
 function selectedDatesForPlan(

src/app/services/calendar-service.spec.ts

@@ -277,7 +277,9 @@ describe('CalendarService', () => {
 				typeof value === 'object' && value ? Object.freeze(value) : value,
 			);
 			const projection = service.project(frozen);
-			expect(projection.horizon.end).toBe('2024-04-04');
+			expect(projection.selections.dayTypes(new Date('2024-04-03T00:00:00'))).toContain(
+				DayType.RTT,
+			);
 			expect(frozen).toEqual(before);
 		});
 	});

src/app/services/calendar-service.ts

@@ -53,7 +53,6 @@ export function calendarProjectionHorizon(plan: CalendarPlan) {
 }
 
 export interface CalendarProjection {
-	horizon: { start: string; end: string };
 	months: { monthIndex: number; year: number }[];
 	selections: SelectedDates;
 }
@@ -147,7 +146,7 @@ export class CalendarService {
 				),
 			);
 		}
-		return { horizon, months, selections: new SelectedDates(ranges) };
+		return { months, selections: new SelectedDates(ranges) };
 	}
 
 	getHolidays(startYear = this.dateCache.getCurrentYear(), lastYear = startYear + 1): Date[] {