frontend: add non-clickable section headers to the sidebar

[Soli0222]

Summary

This PR adds support for non-clickable sidebar section headers that plugins can register through registerSidebarEntry.

Sidebar entries with entryType: 'subheader' are rendered as section headers. Existing sidebar entries continue to render as clickable links by default, so existing registerSidebarEntry usage remains compatible.

Related Issue

Fixes #6107

Changes

• Added an optional entryType field to sidebar entries.
• Added support for entryType: 'subheader' in SidebarItem.
• Render subheader entries as MUI ListSubheader when the sidebar is expanded.
• Render subheader entries as a Divider when the sidebar is collapsed.
• Added optional sx style overrides for subheader entries.
• Updated registerSidebarEntry so plugins can pass entryType and sx.
• Added tests for subheader rendering, collapsed sidebar behavior, hidden entries, and style overrides.
• Added Storybook examples for default, collapsed, and styled section headers.
• Updated the sidebar example plugin to show a section header followed by flat link entries at the same level.

Steps to Test

1. Register a regular sidebar entry without entryType and confirm it still renders as a clickable link.

   registerSidebarEntry({
     parent: null,
     name: 'traces',
     label: 'Traces',
     url: '/traces',
     icon: 'mdi:chart-timeline-variant',
   });

2. Register a sidebar entry with entryType: 'subheader' and confirm it renders as a non-clickable section header.

   registerSidebarEntry({
     parent: null,
     name: 'observability-section',
     label: 'Observability',
     entryType: 'subheader',
   });

3. Collapse the sidebar and confirm the section header is rendered as a divider instead of text.

4. Run the focused frontend checks.

   npm run frontend:lint
   npm run frontend:tsc
   cd frontend
   npm test -- --run src/components/Sidebar/SidebarItem.test.tsx src/components/Sidebar/useSidebarItems.test.tsx
   npm test -- --run src/storybook.test.tsx -t "Sidebar/SidebarItem"

Screenshots (if applicable)

Notes for the Reviewer

• This change is opt-in. Existing sidebar entries are treated as entryType: 'link' by default.

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