Add bilingual day-label headers with Latin weekday/season/ordinal vocabulary

Builds every day label as a {en, la} pair (weekdays, ranks, season/ordinal
phrasing authored in Latin; proper names without an authored Latin form
fall back to their English string) and merges the date+day-label into one
header in the day-nav bar.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ui/day-nav.ts

@@ -1,13 +1,28 @@
 import { getState, shiftDate, goToToday } from '../app-state';
-import { formatDateLong } from './format';
+import { resolveDay } from '../calendar';
+import { getDayLabel } from '../calendar/day-label';
+import { formatDateLong, renderBilingual } from './format';
 
+/** Renders navigation plus the merged "when" header: the civil date/
+ * weekday and the liturgical day-being-celebrated, together in one place
+ * — previously the date lived here while the day-being-celebrated text
+ * was repeated separately inside every hour's own view. The day-being-
+ * celebrated text is hour-independent (the same LiturgicalDay for every
+ * hour on a given date), so it's resolved directly here via `resolveDay`
+ * rather than through any particular hour's `resolveOrdo`. */
 export function renderDayNav(container: HTMLElement): void {
-  const { date } = getState();
+  const { date, languages } = getState();
+  const day = resolveDay(date);
+  const dateLabel = formatDateLong(date);
+  const dayLabel = getDayLabel(day);
   container.innerHTML = `
     <nav class="day-nav" aria-label="Day navigation">
       <button type="button" class="day-nav-btn" data-action="prev" aria-label="Previous day">&larr;</button>
       <button type="button" class="day-nav-btn day-nav-today" data-action="today">Today</button>
-      <span class="day-nav-date">${formatDateLong(date)}</span>
+      <div class="day-nav-header">
+        ${renderBilingual(dateLabel, languages, 'day-nav-date')}
+        ${renderBilingual(dayLabel, languages, 'day-nav-label')}
+      </div>
       <button type="button" class="day-nav-btn" data-action="next" aria-label="Next day">&rarr;</button>
     </nav>
   `;