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)/news/[slug]/page.tsx

@@ -1,4 +1,5 @@
 import type { Metadata } from "next";
+import { formatDate } from "@/lib/format-date";
 import Link from "next/link";
 import { notFound } from "next/navigation";
 import { getTranslations } from "next-intl/server";
@@ -45,9 +46,6 @@ const REACTIONS: { key: string; emoji: string }[] = [
 	{ key: "wow", emoji: "😮" },
 ];
 
-function formatDate(d: Date | null | undefined): string {
-	return d ? d.toISOString().slice(0, 10) : "";
-}
 
 export default async function ArticlePage({
 	params,
@@ -262,7 +260,7 @@ export default async function ArticlePage({
 											<span>{t("userNumber", { id: c.userId })}</span>
 										)}
 										{c.createdAt ? (
-											<span> · {formatDate(c.createdAt)}</span>
+											<span> · {formatDate(c.createdAt, "date", "")}</span>
 										) : null}
 									</p>
 									<p style={{ margin: 0, whiteSpace: "pre-wrap" }}>

src/app/(site)/news/page.tsx

@@ -1,4 +1,5 @@
 import Image from "next/image";
+import { formatDate } from "@/lib/format-date";
 import Link from "next/link";
 import { getTranslations } from "next-intl/server";
 import { ContentCard, EmptyState } from "@/components/public/ui";
@@ -9,9 +10,6 @@ export const revalidate = 300;
 
 export const metadata = { title: "News" };
 
-function formatDate(d: Date | null): string {
-	return d ? d.toISOString().slice(0, 10) : "";
-}
 
 export default async function NewsPage() {
 	const t = await getTranslations("pages.news");
@@ -99,7 +97,7 @@ export default async function NewsPage() {
 										className="muted"
 										style={{ margin: "0.35rem 0 0.6rem", fontSize: "0.78rem" }}
 									>
-										{formatDate(a.createdAt)}
+										{formatDate(a.createdAt, "date", "")}
 									</p>
 									<p
 										style={{