fix(17): IN-07 memoize Intl.DateTimeFormat timezone resolution

apps/pwa/src/components/CalendarShell.tsx

@@ -153,7 +153,12 @@ export function CalendarShell() {
   // IANATimezone (the config's timezone type) is declared but not exported by @schedule-x/calendar,
   // so derive it from useCalendarApp's config parameter rather than importing it.
   type SxTimeZone = NonNullable<Parameters<typeof useCalendarApp>[0]['timezone']>;
-  const displayTimeZone: SxTimeZone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+  // IN-07: memoize so this stable value never changes identity across renders —
+  // it feeds the useCalendarApp config the file works hard to keep stable.
+  const displayTimeZone = useMemo<SxTimeZone>(
+    () => Intl.DateTimeFormat().resolvedOptions().timeZone,
+    [],
+  );
 
   // useCalendarApp — config is stable; plugins passed as second argument
   const calendar = useCalendarApp(