feat(clients): allow attaching clients to socks/mixed inbounds

The client form, bulk-add form and the inbound-side attach entry all
filtered attachable inbounds by a protocol whitelist that lacked
socks/mixed (inherited from upstream), so SK5 inbounds could never be
picked as a client's associated inbound even though the backend
accepts the association. Add 'socks' and 'mixed' to all three gates.
The client record's existing password field is the socks credential;
no new credential shape is introduced.

frontend/src/pages/clients/ClientFormModal.tsx

@@ -61,6 +61,8 @@ const MULTI_CLIENT_PROTOCOLS = new Set([
   'wireguard',
   'mtproto',
   'amneziawg',
+  'socks',
+  'mixed',
 ]);
 
 const CLIENT_FORM_MODAL_Z_INDEX = 1000;