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.

tests/hours/compline.test.ts

@@ -85,11 +85,12 @@ describe('resolveOrdo("compline", ...)', () => {
     expect(last?.kind === 'preces' ? last.text.text.en : undefined).toContain('Hail holy Queen');
   });
 
-  it('anticipates Advent I: the Saturday evening before switches to the Alma Redemptoris Mater', () => {
+  it('anticipates Advent I: the Saturday evening before switches to the Alma Redemptoris Mater and shows the anticipated day label', () => {
     // Nov 30, 2025 is Advent I Sunday.
     const eve = resolveOrdo('compline', '2025-11-29');
     const last = eve.parts[eve.parts.length - 1];
     expect(last?.kind === 'preces' ? last.label : undefined).toBe('Alma Redemptoris Mater');
+    expect(eve.dayLabel).toBe('The 1st Sunday of Advent');
 
     const dayBefore = resolveOrdo('compline', '2025-11-28');
     const lastBefore = dayBefore.parts[dayBefore.parts.length - 1];