feat: Support refreshed BHE sidebar in shared navigation BED-9110

[justin-prime1]

Summary

• add a shared visualRefresh path for BHE main and secondary navigation
• preserve the existing navigation as the default for BHCE and other consumers
• add refreshed hierarchy, collapsed behavior, footer treatment, route semantics, and accessible icon-only names

Product boundary

This does not add a BHCE setting, feature flag, configuration option, or user-facing activation path. BHCE continues to omit visualRefresh and retains its existing navigation experience.

Validation

• just prepare-for-codereview
• 14 focused MainNav and SubNav component tests
• shared UI lint, type, and formatting checks
• BHE light/dark Playwright regression covering expand/collapse, keyboard focus, fixed logo geometry, clipping, and a 640px 200%-equivalent viewport

Accessibility

• preserves accessible names for collapsed icon-only links and actions
• exposes aria-current for selected refreshed routes
• exposes aria-expanded for refreshed subnavigation triggers
• verifies keyboard toggle operation and visible focus

Dependency

Stacked on the BHCE typography adoption branch and PR:

• base: feature/bhce-typography-adoption
• PR: feat: Adopt refreshed typography in BHCE BED-9083 #3068

Jira

BED-9110

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