luci-mod-network: DHCPv6-PD cannot be disabled through LuCI

[evilnull]

Is there an existing issue for this?

• I have searched among all existing issues (including closed issues)

screenshots or captures

No response

Actual behaviour

In LuCI, the DHCPv6-PD option cannot be disabled because unchecking it removes the UCI option instead of setting it to 0.

The option is removed completely instead of being set to: option dhcpv6_pd '0'
Since odhcpd defaults dhcpv6_pd to 1 when the option is absent, DHCPv6-PD remains enabled.
Therefore, LuCI cannot disable DHCPv6-PD.

Expected behaviour

Unchecking DHCPv6-PD should write: option dhcpv6_pd '0'
When the option is absent, LuCI should also display it as enabled, matching the odhcpd default.

Steps to reproduce

1. Set the option explicitly:

   uci set dhcp.lan.dhcpv6_pd='1'
   uci commit dhcp

2. Open:
   Network → Interfaces → LAN → DHCP Server → DHCPv6 Settings

3. Uncheck DHCPv6-PD and click Save & Apply.

4. Check the value:
   uci -q get dhcp.lan.dhcpv6_pd

Additional Information

{
	"kernel": "6.12.94",
	"hostname": "OpenWrt",
	"system": "QEMU Virtual CPU version 2.5+",
	"model": "QEMU Standard PC (Q35 + ICH9, 2009)",
	"board_name": "qemu-standard-pc-q35-ich9-2009",
	"rootfs_type": "ext4",
	"release": {
		"distribution": "OpenWrt",
		"version": "25.12.5",
		"firmware_url": "https://downloads.openwrt.org/",
		"revision": "r33051-f5dae5ece4",
		"target": "x86/64",
		"description": "OpenWrt 25.12.5 r33051-f5dae5ece4",
		"builddate": "1782737960"
	}
}

What browsers do you see the problem on?

Chrome

Relevant log output