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/styles.css

@@ -165,12 +165,26 @@ button:focus-visible {
   cursor: pointer;
 }
 
-.day-nav-date {
-  font-weight: 600;
+.day-nav-header {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 0.15rem;
   min-width: 16rem;
   text-align: center;
 }
 
+.day-nav-date {
+  font-weight: 600;
+}
+
+.day-nav-label {
+  font-family: var(--font-ui);
+  font-size: 0.85rem;
+  font-style: italic;
+  color: var(--color-brass);
+}
+
 /* --------------------------------------------------------------------------
    Layout: hour list + hour view
    -------------------------------------------------------------------------- */
@@ -261,14 +275,6 @@ 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;