fix: Admin Insights actual crash — function props across server/client boundary (v0.55.3)

Server logs (thanks to the user pulling them) showed the real error:
"Functions cannot be passed directly to Client Components" — the server
component page was passing formatShortDate/formatMonth as a `formatDate`
prop into TimeSeriesChart ("use client"). Functions aren't serializable
across the RSC boundary; the two previous fixes (query hardening,
Promise.allSettled) were real improvements but not the actual cause of
the reported crash.

TimeSeriesChart now takes a plain `dateFormat: "day" | "month"` string
and formats internally — BarChart was never affected (its formatValue
prop is only ever used via its own default, never passed from the page).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/admin/insights/page.tsx

@@ -34,11 +34,6 @@ function lastNMonths(n: number): string[] {
   return out;
 }
 
-function formatMonth(m: string) {
-  const [y, mo] = m.split("-");
-  return new Date(Number(y), Number(mo) - 1, 1).toLocaleDateString(undefined, { month: "short", year: "2-digit" });
-}
-
 export default async function AdminInsightsPage() {
   const since = new Date();
   since.setDate(since.getDate() - DAYS);
@@ -128,7 +123,7 @@ export default async function AdminInsightsPage() {
             <CardDescription>Daily, last {DAYS} days</CardDescription>
           </CardHeader>
           <CardContent>
-            <TimeSeriesChart data={signupSeries} formatDate={formatShortDate} />
+            <TimeSeriesChart data={signupSeries} dateFormat="day" />
           </CardContent>
         </Card>
 
@@ -168,7 +163,7 @@ export default async function AdminInsightsPage() {
             <CardDescription>Monthly total across all users, last 6 months</CardDescription>
           </CardHeader>
           <CardContent>
-            <TimeSeriesChart data={usageSeries} formatDate={formatMonth} />
+            <TimeSeriesChart data={usageSeries} dateFormat="month" />
           </CardContent>
         </Card>