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/api-keys/page.tsx

@@ -1,4 +1,5 @@
 import { getTranslations } from "next-intl/server";
+import { formatDate } from "@/lib/format-date";
 import {
 	createApiKey,
 	deleteApiKey,
@@ -23,9 +24,6 @@ type ApiKey = {
 	createdAt: Date | null;
 };
 
-function formatDate(d: Date | null): string {
-	return d ? d.toISOString().slice(0, 16).replace("T", " ") : "\u2014";
-}
 
 function maskKey(key: string): string {
 	return key.length > 8 ? `${key.slice(0, 8)}\u2026` : key;
@@ -204,10 +202,10 @@ export default async function AdminRadioApiKeysPage() {
 											{k.rateLimit}/min
 										</td>
 										<td className="text-sm theme-text-muted dark:theme-text-muted whitespace-nowrap">
-											{formatDate(k.lastUsedAt)}
+											{formatDate(k.lastUsedAt, "datetime", "\u2014")}
 										</td>
 										<td className="text-sm theme-text-muted dark:theme-text-muted whitespace-nowrap">
-											{formatDate(k.expiresAt)}
+											{formatDate(k.expiresAt, "datetime", "\u2014")}
 										</td>
 										<td>
 											<span