Reapply "Add missing admin action files and navigation links"

This reverts commit 4d515bc400.

src/app/admin/rooms/_lib/load-room.ts

@@ -0,0 +1,15 @@
+import { notFound } from 'next/navigation'
+import { prisma } from '@/lib/prisma'
+
+export async function loadRoom(id: string) {
+  const roomId = parseInt(id, 10)
+  if (Number.isNaN(roomId)) notFound()
+
+  const room = await prisma.room.findUnique({
+    where: { id: roomId },
+    include: { owner: { select: { id: true, username: true, look: true } } },
+  })
+
+  if (!room) notFound()
+  return room
+}