fix: wire DM file attachments through the full send/fetch/broadcast chain

DM uploads silently failed because the 5-point chain (types, frontend,
POST, GET, WS broadcast) was never wired for attachments. Added
buildDmMessageWithUser/getDmMessageWithUser helpers mirroring the server
channel pattern, and plumbed attachmentIds + replyToId through all DM
code paths.

packages/web/src/stores/chatStore.ts

@@ -166,7 +166,7 @@ export const useChatStore = create<ChatState>((set, get) => ({
 
     try {
       if (isDm) {
-        await api.dm.sendMessage(channelId, { content });
+        await api.dm.sendMessage(channelId, { content, attachments: attachmentIds, replyToId });
       } else {
         await api.channels.sendMessage(channelId, { content, attachments: attachmentIds, replyToId });
       }