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/lib/format-date.ts

@@ -0,0 +1,36 @@
+export type DateFormatVariant = "date" | "datetime" | "datetime-seconds";
+
+const FALLBACKS: Record<DateFormatVariant, string> = {
+	date: "",
+	datetime: "—",
+	"datetime-seconds": "—",
+};
+
+/**
+ * Format a Date for display. Kept as a single shared helper so the various
+ * admin/site pages no longer each redefine their own copy.
+ *
+ * - "date"            → 2026-07-18
+ * - "datetime"        → 2026-07-18 18:30
+ * - "datetime-seconds"→ 2026-07-18 18:30:45
+ *
+ * `fallback` overrides the default em-dash / empty string when the value is
+ * null/undefined.
+ */
+export function formatDate(
+	d: Date | null | undefined,
+	variant: DateFormatVariant = "datetime",
+	fallback?: string,
+): string {
+	if (!d) return fallback ?? FALLBACKS[variant];
+	const iso = d.toISOString();
+	switch (variant) {
+		case "date":
+			return iso.slice(0, 10);
+		case "datetime-seconds":
+			return iso.slice(0, 19).replace("T", " ");
+		case "datetime":
+		default:
+			return iso.slice(0, 16).replace("T", " ");
+	}
+}