Fix WordPress Mobile Header Issues

On my WordPress site the header looks fine on desktop but falls apart on phones and tablets. The header image refuses to resize, so it either gets cropped or pushes the rest of the content down the page. At the same time the main navigation misbehaves: some items vanish or wrap awkwardly, and the menu as a whole never collapses into a clean mobile-friendly version.

I need the following resolved:

• Header image should scale fluidly across common breakpoints without distortion or cropping.
• Navigation must display every menu item and switch to a responsive layout (hamburger or similar) when screen width demands it.

Clean, lightweight CSS (or theme-compatible adjustments), no hardcoding that will break on updates. After the fix I’ll test with Chrome DevTools and real devices; everything should remain fully editable through the WordPress customizer.