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".

frontend/src/pages/clients/ClientFormModal.tsx

@@ -132,6 +132,7 @@ type Values = ClientFormValues & {
   wgAllowedIPs: string;
   awgAllowedIPs: string;
   awgForwardedPorts: string;
+  wgKeepAlive: number;
   secret: string;
   adTag: string;
 };
@@ -168,6 +169,7 @@ const EMPTY: Values = {
   wgAllowedIPs: '',
   awgAllowedIPs: '',
   awgForwardedPorts: '',
+  wgKeepAlive: 25,
   secret: '',
   adTag: '',
 };
@@ -378,6 +380,7 @@ export default function ClientFormModal({
         wgAllowedIPs: wgTunnelIPs ?? client.allowedIPs ?? '',
         awgAllowedIPs: awgTunnelIPs ?? client.allowedIPs ?? '',
         awgForwardedPorts: client.forwardedPorts || '',
+        wgKeepAlive: client.keepAlive ?? 0,
         secret: client.secret || '',
         adTag: client.adTag || '',
       };
@@ -693,6 +696,7 @@ export default function ClientFormModal({
       // so both protocols share this one field set — see wgPrivateKey etc.
       // below and the AmneziaWG-labeled variants of the same inputs.
       clientPayload.privateKey = values.wgPrivateKey;
+      clientPayload.keepAlive = values.wgKeepAlive;
       clientPayload.publicKey = values.wgPublicKey;
       if (values.wgPreSharedKey) {
         clientPayload.preSharedKey = values.wgPreSharedKey;
@@ -1271,6 +1275,13 @@ export default function ClientFormModal({
                               <Input placeholder="10.8.1.2/32" />
                             </FormField>
                           )}
+                          <FormField
+                            name="wgKeepAlive"
+                            label={t('pages.clients.tunnelKeepAlive')}
+                            extra={t('pages.clients.tunnelKeepAliveHint')}
+                          >
+                            <InputNumber min={0} max={65535} style={{ width: '100%' }} />
+                          </FormField>
                           {showAmneziawg && (
                             <FormField
                               name="awgForwardedPorts"