fix(app-page): clear lint regression from per-tile Suspense scaffolding

The useMemo introduced for the timezone-aware greeting in the W-RSC
per-tile Suspense pass listed [user?.timezone] as its dep array. React
Compiler's preserve-manual-memoization rule rejected the literal
optional-chain expression because the inferred dep is the wider 'user'
object — a less-specific source than the declared property access — so
the compiler refused to optimize the surrounding 1 400-line component
body.

Lift userTimezone into a local before the memo so the inferred dep and
the declared dep agree. No behaviour change; the memo still recomputes
only when the user's timezone string changes.

src/app/page.tsx

@@ -573,9 +573,10 @@ export default function DashboardPage() {
   // changes when the user's timezone changes; recomputing it every
   // render churned the parts-array and walked the format ICU twice per
   // render across a 1 400-line component body.
+  const userTimezone = user?.timezone;
   const hour = useMemo(
-    () => (user?.timezone ? getHourForTimeZone(user.timezone) : null),
-    [user?.timezone],
+    () => (userTimezone ? getHourForTimeZone(userTimezone) : null),
+    [userTimezone],
   );
   const timeGreeting =
     hour == null