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/banners/page.tsx

@@ -1,4 +1,5 @@
 import NextImage from "next/image";
+import { formatDate } from "@/lib/format-date";
 import { getTranslations } from "next-intl/server";
 import {
 	createRadioBanner,
@@ -21,9 +22,6 @@ type Banner = {
 	createdAt: Date | null;
 };
 
-function formatDate(d: Date | null): string {
-	return d ? d.toISOString().slice(0, 16).replace("T", " ") : "\u2014";
-}
 
 export default async function AdminRadioBannersPage() {
 	const t = await getTranslations("pages.admin.radio");
@@ -180,7 +178,7 @@ export default async function AdminRadioBannersPage() {
 									<strong>#{String(b.id)}</strong>
 									<span className="text-sm theme-text-muted dark:theme-text-muted inline-flex gap-2 items-center">
 										{t("banners.byUser", { id: String(b.userId) })} ·{" "}
-										{formatDate(b.createdAt)} ·{" "}
+										{formatDate(b.createdAt, "datetime", "\u2014")} ·{" "}
 										<span
 											className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${b.isActive ? "bg-[var(--admin-success-subtle)] text-[var(--admin-text)]" : "bg-[var(--admin-error-subtle)] text-[var(--admin-text)]"}`}
 										>