ui: show the "day being celebrated" label

Adds a real label under each hour's heading — "Monday in the 10th week
after Trinity", "The 1st Sunday of Advent" — computed from whichever
LiturgicalDay the hour actually resolved against, which can differ from
the nav date for Compline's evening anticipation.

calendar/day-label.ts combines two things: an ordinal week-within-season
label (pure date arithmetic on the season anchors from calendar/temporal.ts
and calendar/easter.ts — Trinity-counted, not Divinum Officium's own
Pentecost-counted convention; meant to become configurable later via the
same day->id indirection already used for the sanctoral calendar, not
hardcoded forever), and a feast name from calendar/commemorations.ts's
occurrence decision — shown alone if the feast displaces the day outright,
prefixed onto the temporal label if merely commemorated, or omitted
entirely if nothing's occurring.

src/ui/hour-view.ts

@@ -130,6 +130,7 @@ export function renderHourView(container: HTMLElement): void {
   container.innerHTML = `
     <div class="hour-view">
       <h2>${hourLabel(selectedHour)}</h2>
+      ${ordo.dayLabel ? `<p class="day-label">${escapeHtml(ordo.dayLabel)}</p>` : ''}
       ${ordo.parts.map((part) => renderPart(part, languages)).join('')}
     </div>
   `;