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/shared/src/types.ts

@@ -226,6 +226,8 @@ export type ServerEvent =
   | { type: 'voice_status_update'; userId: string; channelId: string; isMuted: boolean; isDeafened: boolean; isCameraOn: boolean; isScreenSharing: boolean }
   | { type: 'dm_channel_created'; dmChannel: DmChannel }
   | { type: 'dm_channel_closed'; dmChannelId: string }
+  | { type: 'dm_member_added'; dmChannelId: string; user: User }
+  | { type: 'dm_member_removed'; dmChannelId: string; userId: string }
   | { type: 'friend_removed'; userId: string }
   | { type: 'channel_created'; channel: Channel; serverId: string }
   | { type: 'channel_updated'; channel: Channel; serverId: string }
@@ -312,6 +314,10 @@ export interface CreateDmRequest {
   userId: string;
 }
 
+export interface AddDmMemberRequest {
+  userId: string;
+}
+
 export interface CreateDmMessageRequest {
   content: string;
 }