refactor: centralize duplicated formatDate helper

21 files defined their own local formatDate (with three different output
formats: date, datetime, datetime-seconds, and varying null fallbacks).
Replace them with a single shared helper at src/lib/format-date.ts that
takes a variant + optional fallback, preserving the exact previous output
per call site (verified identical string results).

Removes ~21 copies of the same logic. photos.tsx and media-grid.tsx keep
their epoch-ms based formatters since those are a different input shape.

Verified: tsc --noEmit clean, full test suite green (301/301).

src/app/(site)/radio/contests/[id]/page.tsx

@@ -1,4 +1,5 @@
 import Link from "next/link";
+import { formatDate } from "@/lib/format-date";
 import { notFound } from "next/navigation";
 import { getTranslations } from "next-intl/server";
 import { ContentCard } from "@/components/public/ui";
@@ -6,9 +7,6 @@ import { prisma } from "@/lib/prisma";
 
 export const revalidate = 300;
 
-function formatDate(d: Date | null | undefined): string {
-	return d ? d.toISOString().slice(0, 19).replace("T", " ") : "";
-}
 
 export default async function RadioContestDetailPage({
 	params,
@@ -63,15 +61,15 @@ export default async function RadioContestDetailPage({
 						<tr>
 							<th>Period</th>
 							<td>
-								{contest.startDate ? formatDate(contest.startDate) : "—"}
+								{contest.startDate ? formatDate(contest.startDate, "datetime-seconds", "") : "—"}
 								{" — "}
-								{contest.endDate ? formatDate(contest.endDate) : "ongoing"}
+								{contest.endDate ? formatDate(contest.endDate, "datetime-seconds", "") : "ongoing"}
 							</td>
 						</tr>
 						<tr>
 							<th>Created</th>
 							<td>
-								{formatDate(contest.createdAt) || (
+								{formatDate(contest.createdAt, "datetime-seconds", "") || (
 									<span className="muted">—</span>
 								)}
 							</td>
@@ -79,7 +77,7 @@ export default async function RadioContestDetailPage({
 						<tr>
 							<th>Updated</th>
 							<td>
-								{formatDate(contest.updatedAt) || (
+								{formatDate(contest.updatedAt, "datetime-seconds", "") || (
 									<span className="muted">—</span>
 								)}
 							</td>