feat(side-menu): animate nested sidebar expand/collapse

[illionillion]

What/Why/How?

This PR adds smooth accordion animation for nested items in the left sidebar menu.

Why:

• current expand/collapse is abrupt
• smoother transitions improve UX without changing navigation logic

How:

• replaced abrupt nested list show/hide behavior with animated height transition
• uses measured scrollHeight for smooth open/close
• keeps root menu behavior unchanged
• preserves accessibility semantics (role="menu" for root, aria-hidden for collapsed nested lists)
• respects prefers-reduced-motion

Reference

Closes #2778

Tests

• Added regression tests:
  • src/components/__tests__/menuItemsA11y.test.ts
• Local checks:
  • npm run bundle ✅
  • npm test ✅
  • npm run e2e ⚠️ fails locally with ERR_INVALID_CHAR on x-cypress-file-path in this environment (workspace path contains non-ASCII characters). CI should validate e2e in standard runner paths.

Screenshots (optional)

Befor

Screencast.from.2026-03-10.00-40-19.webm

After

Screencast.from.2026-03-10.00-43-23.webm

Check yourself

• Code is linted
• Tested
• All new/updated code is covered with tests

[illionillion]

@adamaltman

Friendly ping when you have a moment 🙏
I added a screen recording and kept the scope small (sidebar accordion animation only, with tests).
Happy to make any adjustments if needed.