fix: preserve sidebar collapse state across route changes (Closes #914)

[waterWang]

Summary

Fixes the sidebar (MobileNav drawer) collapse state being lost on route change. The sidebar now preserves its open/closed state when navigating between SPA routes.

Changes

• Removed the render-phase close logic that was forcing the sidebar to close on every route change
• The sidebar's open/closed state is now persisted via sessionStorage across both SPA route changes and full page reloads

• Added in to fix test isolation (pre-existing issue)

• Added in to fix test isolation
• Added new test: preserves sidebar collapse state across route changes — verifies the drawer stays open after navigating via a desktop nav link

Testing

• Lint passes
• Tests pass (11/12 Layout tests, 19/19 MobileNav tests — the 1 Layout failure is pre-existing 'renders theme toggle button')
• Build compiles (pre-existing tsc error in Attestations.tsx is unrelated)

Closes #914