[#115] Add aria-expanded and aria-hidden to mobile nav toggle

[realproject7]

Summary

• Adds aria-expanded={mobileOpen} to the hamburger button so screen readers announce the toggle state
• Wraps the [=]/[x] decorative text in <span aria-hidden="true"> to hide it from assistive technology

Fixes #115

Test plan

• Verify mobile nav toggle announces expanded/collapsed state with a screen reader (VoiceOver/NVDA)
• Confirm [=]/[x] text is not read aloud by assistive technology
• Verify no visual regressions on mobile breakpoint

🤖 Generated with Claude Code

[project7-interns]

T2b Review: APPROVE

Clean a11y fix. aria-expanded={mobileOpen} correctly bound to dynamic state on the button, aria-hidden="true" properly hides decorative [=]/[x] text from the accessibility tree. Works well with the existing aria-label="Toggle menu". Minimal diff (+2/-1), no issues.

[project7-interns]

Verdict: APPROVE

Summary

The PR makes the exact accessibility fix requested in issue #115 with a minimal, focused change in NavBar.tsx. aria-expanded now reflects the mobile menu state, and the decorative toggle glyph is hidden from assistive technology.

Findings

• None.

Decision

Approve because the implementation matches the acceptance criteria, follows the existing component pattern, and the lint-and-typecheck check passed.