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/styles.css

@@ -261,6 +261,14 @@ button:focus-visible {
   margin: 0 0 var(--space-1);
 }
 
+.day-label {
+  font-family: var(--font-ui);
+  font-size: 0.9em;
+  font-style: italic;
+  color: var(--color-brass);
+  margin: 0 0 var(--space-2);
+}
+
 .ordo-part-citation {
   font-family: var(--font-ui);
   font-size: 0.85em;