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 {
-		horizon,
-		months,
-		selections: selectedDatesForPlan(value, includeHoliday),
-	};
+	return { months, selections: selectedDatesForPlan(value, includeHoliday) };
 }
 
 function selectedDatesForPlan(