refactor(calendar): encapsulate calendar projection and selection ranges

CalendarService.project returns the covered months together with the
projected selections, so month enumeration and selected-day rendering
share one projection horizon. SelectedDates is built once from the
complete range list and groups connected periods internally, exposing
read-only selection, boundary and day-type queries.

In declaration entry, months reached by adjacent declared or
non-working days are now shown, matching the projected selections.

Refs #276

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

src/app/components/calendar-month/calendar-month.ts

@@ -184,14 +184,10 @@ export class CalendarMonth implements OnInit {
 
 	getDayTooltip(day: Date): string | null {
 		const previewed = this.declarationPreview?.dates.has(this.getDateKey(day));
-		const tooltips: string[] = this.selectedDates.datesSelected
-			.filter(
-				(dateSelected) =>
-					dateSelected.isSelected(day) &&
-					dateSelected.type &&
-					(!previewed || !['cp', 'rtt', 'other'].includes(dateSelected.type)),
-			)
-			.map((dateSelected) => this.translationService.getDayTypeLabel(dateSelected.type!));
+		const tooltips: string[] = this.selectedDates
+			.dayTypes(day)
+			.filter((type) => !previewed || !['cp', 'rtt', 'other'].includes(type))
+			.map((type) => this.translationService.getDayTypeLabel(type));
 		const declaredType = this.getDeclaredType(this.getDateKey(day));
 		if (declaredType)
 			tooltips.push(