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/hours/types.ts

@@ -135,4 +135,12 @@ export interface ResolvedOrdo {
   parts: ResolvedPart[];
   /** Set when this hour hasn't been built yet — UI shows "coming soon" instead of empty content. */
   notImplemented?: true;
+  /**
+   * "Monday in the 10th week after Trinity", "St. Ereden — ...", etc. —
+   * see calendar/day-label.ts. Computed from whichever LiturgicalDay the
+   * hour actually resolved against, which can differ from `date` itself
+   * (Compline may anticipate tomorrow — see calendar/vespers.ts). Absent
+   * on not-yet-built hours.
+   */
+  dayLabel?: string;
 }