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/shouts/page.tsx

@@ -1,4 +1,5 @@
 import { getTranslations } from "next-intl/server";
+import { formatDate } from "@/lib/format-date";
 import { postShout } from "@/actions/radio-shouts";
 import { ContentCard, EmptyState } from "@/components/public/ui";
 import { auth } from "@/lib/auth";
@@ -8,9 +9,6 @@ import { siteSettings } from "@/lib/services/site-settings";
 
 export const dynamic = "force-dynamic";
 
-function formatDate(d: Date | null | undefined): string {
-	return d ? d.toISOString().slice(0, 16).replace("T", " ") : "";
-}
 
 export default async function RadioShoutsPage() {
 	const t = await getTranslations("pages.radioShouts");
@@ -124,7 +122,7 @@ export default async function RadioShoutsPage() {
 									<div style={{ flex: 1, minWidth: 0 }}>
 										<p style={{ margin: "0 0 0.25rem" }}>
 											<strong>{author?.username ?? `User #${s.userId}`}</strong>{" "}
-											<span className="muted">{formatDate(s.createdAt)}</span>
+											<span className="muted">{formatDate(s.createdAt, "datetime", "")}</span>
 										</p>
 										<p style={{ margin: 0 }}>{s.message}</p>
 									</div>