feat: add group DM support (3-10 members)

Unlock group DMs by removing the 2-member assumption across the stack.
No schema migration needed — dm_members junction table already supports
N members. Includes dedup bug fix, add/leave member endpoints, late-join
call support, group-aware sidebar rendering, and AddDmMemberModal.

packages/web/src/hooks/useWebSocket.ts

@@ -261,6 +261,18 @@ function handleEvent(event: ServerEvent): void {
       removeDmChannel(event.dmChannelId);
       break;
 
+    case 'dm_member_added': {
+      const { addDmMember } = useServerStore.getState();
+      addDmMember(event.dmChannelId, event.user);
+      break;
+    }
+
+    case 'dm_member_removed': {
+      const { removeDmMember } = useServerStore.getState();
+      removeDmMember(event.dmChannelId, event.userId);
+      break;
+    }
+
     case 'friend_removed': {
       const { removeFriendLocally } = useSocialStore.getState();
       removeFriendLocally(event.userId);