Add Mobile Menu Visual Regression Test

[joelpittet]

Description

Add a visual regression test for the mobile menu functionality to ensure mobile navigation remains consistent across theme updates.

Acceptance Criteria

• Test mobile menu toggle button visibility
• Test mobile menu expanded state
• Test mobile menu collapsed state
• Screenshots captured at mobile viewport (375px)
• Test passes consistently

Technical Notes

• Use page.setViewportSize({ width: 375, height: 667 })
• Test both open/closed states of mobile navigation
• Ensure mobile menu is actually visible before testing

Dependencies

• Mobile menu functionality working properly
• Navigation elements visible on test pages