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/stores/uiStore.ts

@@ -11,6 +11,7 @@ type ModalType =
   | 'serverSettings'
   | 'imagePreview'
   | 'newDm'
+  | 'addDmMember'
   | null;
 
 interface UIState {