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/admin/radio/history/page.tsx

@@ -1,4 +1,5 @@
 import { getTranslations } from "next-intl/server";
+import { formatDate } from "@/lib/format-date";
 import { StatusCard } from "@/components/admin/dashboard";
 import { prisma } from "@/lib/prisma";
 
@@ -14,9 +15,6 @@ type History = {
 	notes: string | null;
 };
 
-function formatDate(d: Date | null): string {
-	return d ? d.toISOString().slice(0, 16).replace("T", " ") : "\u2014";
-}
 
 function formatDuration(start: Date, end: Date | null): string {
 	if (!end) return "\u2014";
@@ -137,12 +135,12 @@ export default async function AdminRadioHistoryPage() {
 									</td>
 									<td>{h.showName ?? "\u2014"}</td>
 									<td className="text-sm theme-text-muted dark:theme-text-muted whitespace-nowrap">
-										{formatDate(h.startedAt)}
+										{formatDate(h.startedAt, "datetime", "\u2014")}
 									</td>
 									<td>
 										{h.endedAt ? (
 											<span className="text-sm theme-text-muted dark:theme-text-muted">
-												{formatDate(h.endedAt)}
+												{formatDate(h.endedAt, "datetime", "\u2014")}
 											</span>
 										) : (
 											<span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-[var(--admin-success-subtle)] text-[var(--admin-text)]">