feat(messaging-ui): import a channel's roster from the settings panel

Phase C of channel-roster import. Adds the optional addMembers(threadId, userIds)
adapter seam (+ BulkAddResult) and, in ConversationSettings, a '#' mode on the
existing Add-people box that lists the channels you belong to — public AND
private, since the source list is your own membership-scoped conversation list.

Picking a channel STAGES the import (reads its roster, diffs against who is
already here) and shows a confirm — 'Add 9 people from #design? (3 already here)'
— so an administrative action never fires on a stray click. The result line
reports added / already-a-member / failed. Absent addMembers => the affordance is
hidden entirely and '#' is just a search string.

Bumped to 0.1.8. SDK suite: 15 files / 78 tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@insignia/iios-messaging-ui",
-  "version": "0.1.7",
+  "version": "0.1.8",
   "type": "module",
   "main": "dist/index.js",
   "module": "dist/index.js",