dhcpd: expose custom DHCPv4 options in web interface

[Sil3ntVip3r]

Summary

• expose DHCPv4 custom options through the status and configuration API while preserving existing YAML values when the new field is omitted
• add a web-interface editor with format help and an explicit warning that a custom DNS option can bypass filtering
• validate submitted options before stopping the current DHCP server, document supported formats, and cover PXE options 66 and 67

Compatibility

• omitting v4.options preserves existing custom options
• sending [] explicitly clears them
• invalid options return a bad-request response without changing the running configuration

Verification

• make go-check
• make go-build go-os-check
• make js-lint md-lint txt-lint
• NODE_OPTIONS=--localstorage-file=/private/tmp/adguardhome-vitest-localstorage npm run check
• npm run translations:check
• npm run locales:check
• npm run build-prod

Closes #7036.