The menu item, backend endpoint (POST /panel/api/inbounds/:id/copyClients),
and i18n keys were already in place after the Vue3 migration, but the modal
itself was never ported — clicking the menu just toasted "coming soon".
Adds CopyClientsModal.vue: source inbound dropdown (multi-user inbounds
except the target), per-client checkbox selection via a-table row-selection,
optional Flow override when the target supports TLS flow, and result toasts
for added/skipped/errors.
MHSanaei