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.

internal/web/runtime/local.go

@@ -241,7 +241,7 @@ func (l *Local) AddClient(ctx context.Context, ib *model.Inbound, client model.C
 		"publicKey":    client.PublicKey,
 		"allowedIPs":   client.AllowedIPs,
 		"preSharedKey": client.PreSharedKey,
-		"keepAlive":    wgKeepAlive(client.KeepAlive),
+		"keepAlive":    wgKeepAlive(client.KeepAliveSeconds()),
 	}
 	return l.AddUser(ctx, ib, user)
 }
@@ -282,7 +282,7 @@ func (l *Local) UpdateUser(ctx context.Context, ib *model.Inbound, oldEmail stri
 		"publicKey":    payload.PublicKey,
 		"allowedIPs":   payload.AllowedIPs,
 		"preSharedKey": payload.PreSharedKey,
-		"keepAlive":    wgKeepAlive(payload.KeepAlive),
+		"keepAlive":    wgKeepAlive(payload.KeepAliveSeconds()),
 	}
 	return l.AddUser(ctx, ib, user)
 }