fix(ui): keep the empty-group placeholder legible in dark mode

The "no group" em dash in the clients table and in two client-picker
modals was drawn with an inline color: rgba(0,0,0,0.45). The panel renders
every page under antd's darkAlgorithm as well, so on a dark container that
near-black placeholder is effectively invisible.

All three now use Typography.Text type="secondary", the idiom the rest of
the panel already uses for muted text — GroupAddClientsModal itself uses it
58 lines further down.

Mid-greys such as #888 elsewhere in the panel stay legible in both themes
and are deliberately left alone.

frontend/src/pages/clients/ClientsPage.tsx

@@ -23,6 +23,7 @@ import {
   Table,
   Tag,
   Tooltip,
+  Typography,
   message,
 } from 'antd';
 import type { ColumnsType, TableProps } from 'antd/es/table';
@@ -1091,7 +1092,7 @@ export default function ClientsPage() {
         width: 130,
         hidden: allGroups.length === 0,
         render: (_v, record) => {
-          if (!record.group) return <span style={{ color: 'rgba(0,0,0,0.45)' }}>—</span>;
+          if (!record.group) return <Typography.Text type="secondary">—</Typography.Text>;
           const isActive = filters.groups.includes(record.group);
           return (
             <Tag

frontend/src/pages/groups/GroupAddClientsModal.tsx

@@ -82,7 +82,7 @@ export default function GroupAddClientsModal({
           g ? (
             <Tag color="geekblue">{g}</Tag>
           ) : (
-            <span style={{ color: 'rgba(0,0,0,0.45)' }}>—</span>
+            <Typography.Text type="secondary">—</Typography.Text>
           ),
       },
       {

frontend/src/pages/inbounds/clients/AttachExistingClientsModal.tsx

@@ -125,7 +125,7 @@ export default function AttachExistingClientsModal({
           group ? (
             <Tag color="geekblue">{group}</Tag>
           ) : (
-            <span style={{ color: 'rgba(0,0,0,0.45)' }}>—</span>
+            <Typography.Text type="secondary">—</Typography.Text>
           ),
       },
       {