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/page.tsx

@@ -1,13 +1,11 @@
 import Link from "next/link";
+import { formatDate } from "@/lib/format-date";
 import { getTranslations } from "next-intl/server";
 import { ContentCard, EmptyState } from "@/components/public/ui";
 import { prisma } from "@/lib/prisma";
 
 export const revalidate = 300;
 
-function formatDate(d: Date | null | undefined): string {
-	return d ? d.toISOString().slice(0, 10) : "";
-}
 
 export default async function RadioContestsPage() {
 	const t = await getTranslations("pages.radioContests");
@@ -67,7 +65,7 @@ export default async function RadioContestsPage() {
 										style={{ margin: 0, fontSize: "0.85rem" }}
 									>
 										{c.startDate
-											? `${formatDate(c.startDate)} — ${c.endDate ? formatDate(c.endDate) : "ongoing"}`
+											? `${formatDate(c.startDate, "date", "")} — ${c.endDate ? formatDate(c.endDate, "date", "") : "ongoing"}`
 											: t("dateUnknown")}
 									</p>
 								</Link>