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/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>
           ),
       },
       {