Add a Files section to room info listing all sent attachments (#33)

Lists files and images actually attached to sent messages in a room,
newest first, with click-through to a lightbox, preview modal, or direct
download depending on type. Queries through messages.image_id/file_id
so an upload that was never sent doesn't show up as a phantom entry.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/api/rooms.ts

@@ -4,6 +4,7 @@ import type {
   MessageFileInfo,
   MyRoomItem,
   Room,
+  RoomAttachment,
   RoomListItem,
   RoomMember,
   RoomRole,
@@ -54,6 +55,10 @@ export function listRoomMembers(roomId: string): Promise<RoomMember[]> {
   return apiFetch<RoomMember[]>(`/api/rooms/${roomId}/members`)
 }
 
+export function listRoomAttachments(roomId: string): Promise<RoomAttachment[]> {
+  return apiFetch<RoomAttachment[]>(`/api/rooms/${roomId}/attachments`)
+}
+
 export function addRoomMember(roomId: string, userId: string): Promise<RoomMember> {
   return apiFetch<RoomMember>(`/api/rooms/${roomId}/members`, {
     method: 'POST',