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.
Muse