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)/u/[username]/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";
@@ -47,9 +48,6 @@ function formatTimestamp(ts: number | null | undefined): string {
 	return new Date(ts * 1000).toISOString().slice(0, 10);
 }
 
-function formatDate(d: Date | null | undefined): string {
-	return d ? d.toISOString().slice(0, 10) : "";
-}
 
 export default async function ProfilePage({
 	params,
@@ -565,7 +563,7 @@ export default async function ProfilePage({
 											<strong>
 												{author?.username ?? t("unknownUser", { id: g.userId })}
 											</strong>{" "}
-											<span className="muted">{formatDate(g.createdAt)}</span>
+											<span className="muted">{formatDate(g.createdAt, "date", "")}</span>
 										</p>
 										<p style={{ margin: 0 }}>{g.message}</p>
 									</div>