fix(clients): let an explicit 0 actually disable PersistentKeepalive

Addresses review feedback on the previous commit.

UpdateInboundClient carries a stored keepalive forward whenever the incoming
one is zero, so the settings JSON and the running peer survive a metadata-only
edit that omits the field. That was a 0 -> 0 no-op while no UI could set a
nonzero value. Now that the client form can, the carry-forward became reachable
in the other direction: a client created at the form's default of 25 could
never be returned to 0, and the hint text shipped to all 13 locales -- "0
disables it" -- described something the backend silently refused. The save even
reported success, because a settings blob that came back byte-identical skips
the transaction entirely.

The zero value cannot carry that distinction, so model.Client.KeepAlive becomes
*int: nil means the field was never sent, &0 means "send no keepalives". The
pointer survives the internal marshal in ClientService.Update, which is where an
explicit 0 was being erased by omitempty before UpdateInboundClient ever saw it.
ClientRecord.KeepAlive stays a plain int -- it is the stored column, where
"unset" has no meaning -- and the conversions bridge the two.

Two tests, both red before this change in the direction they cover: an explicit
0 must reach wg_keep_alive, and an update that omits the field must still leave
a stored 25 alone.

Also adds the output transform every other numeric field in the client form
already has, so a cleared box sends 0 rather than null.

frontend/public/openapi.json

@@ -1147,6 +1147,8 @@
             "type": "string"
           },
           "keepAlive": {
+            "description": "Seconds between PersistentKeepalive packets; 0 sends none, omit to keep the stored value",
+            "nullable": true,
             "type": "integer"
           },
           "limitIp": {

frontend/src/generated/examples.ts

@@ -275,7 +275,7 @@ export const EXAMPLES: Record<string, unknown> = {
     "forwardedPorts": "",
     "group": "",
     "id": "",
-    "keepAlive": 0,
+    "keepAlive": null,
     "limitIp": 0,
     "password": "",
     "preSharedKey": "",

frontend/src/generated/schemas.ts

@@ -1121,6 +1121,8 @@ export const SCHEMAS: Record<string, unknown> = {
         "type": "string"
       },
       "keepAlive": {
+        "description": "Seconds between PersistentKeepalive packets; 0 sends none, omit to keep the stored value",
+        "nullable": true,
         "type": "integer"
       },
       "limitIp": {

frontend/src/generated/types.ts

@@ -271,7 +271,7 @@ export interface Client {
   forwardedPorts?: string;
   group?: string;
   id?: string;
-  keepAlive?: number;
+  keepAlive?: number | null;
   limitIp: number;
   password?: string;
   preSharedKey?: string;

frontend/src/generated/zod.ts

@@ -292,7 +292,7 @@ export const ClientSchema = z.object({
   forwardedPorts: z.string().optional(),
   group: z.string().optional(),
   id: z.string().optional(),
-  keepAlive: z.number().int().optional(),
+  keepAlive: z.number().int().nullable().optional(),
   limitIp: z.number().int(),
   password: z.string().optional(),
   preSharedKey: z.string().optional(),

frontend/src/pages/clients/ClientFormModal.tsx

@@ -1279,6 +1279,7 @@ export default function ClientFormModal({
                             name="wgKeepAlive"
                             label={t('pages.clients.tunnelKeepAlive')}
                             extra={t('pages.clients.tunnelKeepAliveHint')}
+                            transform={{ output: (v) => Number(v) || 0 }}
                           >
                             <InputNumber min={0} max={65535} style={{ width: '100%' }} />
                           </FormField>