Make day-nav/language-toggle real links, group hours into rows

Day navigation (prev/today/next) and the language toggle were plain
buttons wired to click handlers only; switched them to real <a href>
elements (built via router's new exported urlFor) so right-click,
middle-click, and open-in-new-tab work, with a click handler that
still SPA-routes on a plain left-click and lets modified clicks fall
through to normal browser navigation.

Language selection is now reflected in the URL as a ?lang= query
param (parsed/pushed by router.ts, omitted for the default single-
English case to keep that URL clean), so a shared link preserves the
viewer's chosen language(s).

hour-list.ts now groups the 8 hours into three traditional rows
(night office, day hours, evening) instead of one flat wrapped list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014axryJBrRswYh2niA7WCUc

src/app-state.ts

@@ -60,10 +60,14 @@ export function goToToday(): void {
   setDate(todayIso());
 }
 
-export function shiftDate(days: number): void {
-  const d = new Date(`${state.date}T00:00:00Z`);
+export function addDays(date: string, days: number): string {
+  const d = new Date(`${date}T00:00:00Z`);
   d.setUTCDate(d.getUTCDate() + days);
-  setDate(d.toISOString().slice(0, 10));
+  return d.toISOString().slice(0, 10);
+}
+
+export function shiftDate(days: number): void {
+  setDate(addDays(state.date, days));
 }
 
 export function setSelectedHour(hourId: HourId): void {