feat(clients): let admins set PersistentKeepalive on tunnel clients

model.Client already carries KeepAlive, and every AmneziaWG/WireGuard client
config emitter already writes PersistentKeepalive when it is above zero -- but
nothing in the UI could set it, so it stayed 0 and the line was never emitted.

Without it a peer that goes quiet has nothing to trigger a handshake: WireGuard
only initiates when it has data to send. An idle client stays disconnected
after any interruption -- a NAT mapping timing out, a device sleeping, the
panel restarting -- until the user generates traffic themselves.

New clients default to 25, the conventional value, which also keeps the NAT
mapping open. Existing clients keep whatever they have, and 0 remains valid and
means "do not send keepalives".

internal/web/translation/pt-BR.json

@@ -874,6 +874,8 @@
       "amneziaWgAllowedIPsHint": "Deixe vazio para atribuir automaticamente; separe as entradas com vírgulas",
       "amneziaWgForwardedPorts": "Portas encaminhadas",
       "amneziaWgForwardedPortsHint": "Portas/intervalos redirecionados (DNAT) para este cliente, ex. 80, 443, 8000-8100. Deixe vazio se não aplicável.",
+      "tunnelKeepAlive": "Keepalive (seconds)",
+      "tunnelKeepAliveHint": "How often the client sends a keepalive packet. 25 is the usual value: it holds the NAT mapping open and brings an idle peer back on its own after any drop, including a panel restart. 0 disables it — an idle client then stays disconnected until it sends traffic.",
       "amneziaWgConfig": "Configuração do AmneziaWG",
       "mtprotoSecret": "Segredo MTProto",
       "mtprotoSecretHint": "O segredo FakeTLS do cliente. Gere novamente para trocá-lo.",