Allow settings.section registrants to declare a nav icon

[heoo0124]

Summary

Third-party plugins that register a settings.section page always render
with the fallback gear icon in the settings nav rail, because the icon
channel does not exist at any layer: not in the registration options,
not in the ledger projection, and not in the render call. A plugin cannot
fix this on its own — any workaround (patching the host install, DOM
hacks) breaks on every upgrade. This proposal asks for a small,
backward-compatible icon channel on the settings.section registration,
following the icon?: ReactNode prop pattern already established in
ui-primitives.

Real-world case

dsh-plugin-model-modalities registers a settings companion page:

ctx.slots.register({
  name: 'settings.section',
  id: 'modalities',
  order: 20,
  label: () => t('nav'),
  locale: NS,
  inject,
}, ModalitiesSection)