feat(messaging-ui): start direct messages + groups (Slack-style people picker)

Add a 'New message' + on the Direct messages section that opens a people picker
(NewConversation): pick one person -> DM, two or more -> group with an optional
name, via the adapter's existing openThread({participantIds, membership, subject}).
Expose directory() on MessagingAdapter (org people list); MockAdapter implements
it + dedupes 1:1 DMs. 72 tests green.

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

packages/iios-messaging-ui/src/styles.css

@@ -500,6 +500,32 @@
   gap: 5px;
   cursor: pointer;
 }
+/* new conversation (DM / group) picker */
+.miu-newconv {
+  display: flex;
+  flex-direction: column;
+  gap: 10px;
+}
+.miu-person-row {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  padding: 8px 10px;
+  border: 1px solid var(--miu-border);
+  border-radius: 10px;
+  background: var(--miu-panel);
+  cursor: pointer;
+}
+.miu-person-row:hover {
+  border-color: var(--miu-accent);
+}
+.miu-person-row.is-active {
+  border-color: var(--miu-accent);
+  background: color-mix(in srgb, var(--miu-accent) 10%, var(--miu-panel));
+}
+.miu-person-row .miu-browser-name {
+  flex: 1 1 auto;
+}
 .miu-browser-list {
   display: flex;
   flex-direction: column;