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>

tests/calendar/christ-the-king.test.ts

@@ -16,7 +16,7 @@ describe('Christ the King (applyChristTheKing)', () => {
     // already carrying (Ss. Chrysanthus and Daria, Simplex, per the next
     // test below) -- this app's day label now surfaces every commemorated
     // saint, not just the Sunday itself.
-    expect(getDayLabel(day)).toBe('Christ the King (Duplex I Class) — Ss. Chrysanthus and Daria, Martyrs');
+    expect(getDayLabel(day).en).toBe('Christ the King (Duplex I Class) — Ss. Chrysanthus and Daria, Martyrs');
   });
 
   it('preserves a nested commemoration: a Simplex saint already commemorated under the ordinary Sunday stays commemorated', () => {