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/vi-VN.json

@@ -874,6 +874,8 @@
       "amneziaWgAllowedIPsHint": "Để trống để tự động gán; phân tách các mục bằng dấu phẩy",
       "amneziaWgForwardedPorts": "Cổng chuyển tiếp",
       "amneziaWgForwardedPortsHint": "Cổng/dải cổng được chuyển tiếp (DNAT) đến client này, vd. 80, 443, 8000-8100. Để trống nếu không cần.",
+      "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": "Cấu hình AmneziaWG",
       "mtprotoSecret": "Secret MTProto",
       "mtprotoSecretHint": "Secret FakeTLS của client. Tạo lại để thay đổi.",