New components: `SecondaryNav` and `NavigationLayout` for contextual secondary navigation

[zoharma]

What is being proposed?

Two new components, built on top of the existing SidebarNav:

• SecondaryNav: a second, contextual navigation panel (grouped items, optional one-level nesting, optional search, optional title/back header) that sits alongside the primary sidebar. It renders as a fixed side panel on desktop and a temporary overlay drawer on mobile.
• NavigationLayout: a composing layout component that owns SidebarNav + SecondaryNav + main content together, and handles the responsive coordination between the two panels (e.g. only one temporary drawer visible at a time on mobile, back-navigation between them).

Why is this needed?

SidebarNav alone only models a single, flat/nested primary navigation tree. Several Diamond apps need a second level of navigation that's contextual to what's selected in the primary sidebar (e.g. picking Experiments in the sidebar, then navigating between each experiment in a secondary panel), something like SidebarNav's pattern, but for a scoped set of items that changes with context.

Without a shared component, each consuming app would re-implement this panel-plus-drawer pattern, including the fiddly responsive behaviour (mobile drawer stacking, back button wiring, history/popstate handling)
that NavigationLayout centralises.

What will change?

• Introduces two new components: SecondaryNav and NavigationLayout (both already implemented and exported from src/index.ts on this branch).
• No changes to existing component props/behaviour, other than a small addition to SidebarNav (see below) needed for NavigationLayout to drive it.
• New shared type: LinkProps in src/components/navigation/types.ts, reused by SecondaryNav's item definitions.

Interface changes (if any)

// Standalone SecondaryNav
<SecondaryNav
  open={open}
  setOpen={setOpen}
  title="Beamline I03"
  search={{ value, onChange, placeholder: "Search" }}
  groups={[
    {
      subheader: "Experiments",
      items: [
        { id: "exp-1", label: "Experiment 1", linkProps: { href: "/exp-1" }, selected: true },
        {
          id: "exp-2",
          label: "Experiment 2",
          children: [
            { id: "exp-2-a", label: "Run A", linkProps: { href: "/exp-2/a" } },
          ],
        },
      ],
    },
  ]}
/>

// Composed with SidebarNav via NavigationLayout
<NavigationLayout
  navigation={navigation}
  sidebarOpen={sidebarOpen}
  setSidebarOpen={setSidebarOpen}
  secondaryNav={{ title: "Beamline I03", groups }}
  secondaryNavOpen={secondaryNavOpen}
  setSecondaryNavOpen={setSecondaryNavOpen}
>
  {children}
</NavigationLayout>

Breaking change?

• Yes
• No

Next steps

A maintainer will review this issue.
If accepted, it will be marked as accepted and a PR may then be opened.

[zoharma]

@douglaswinter