web, core, desktop: share a note with a group, and Settings → Groups

The Share dialog lists people and groups in one picker and shows a group share
as its name and member count. Settings gains a Groups section for the admin:
create, rename, delete, and add or remove people.

The core client reads the directory's groups and group shares (ShareTarget:
a member or a group); the desktop command takes user_id or group_id.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -5,6 +5,7 @@ import { useConfigStore } from "../stores/config";
 import BaseButton from "../components/BaseButton.vue";
 import InviteList from "../components/InviteList.vue";
 import AccountList from "../components/AccountList.vue";
+import GroupList from "../components/GroupList.vue";
 import { errorMessage } from "../api/errors";
 import { useUiStore } from "../stores/ui";
 
@@ -227,9 +228,10 @@ onMounted(load);
       </div>
     </form>
 
-    <!-- Outside the settings form: each invite and account action saves on its own,
+    <!-- Outside the settings form: each invite, account and group action saves on its own,
          and the form's Save button has nothing to do with them. -->
     <InviteList v-if="items.length" class="mt-10" />
     <AccountList v-if="items.length" class="mt-10" />
+    <GroupList v-if="items.length" class="mt-10" />
   </div>
 </template>