fix(header): improve navbar scroll behavior outside hero section

[harishsehlangia]

Before creating this PR, please confirm the following:

• I have read the ReactPlay Contributor Guidelines and Discussion
• I have reviewed the ReactPlay Code Review Checklist
• I have tested my changes locally and verified there are no new warnings or errors

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First thing, PLEASE READ THIS: ReactPlay Code Review Checklist

Description

The navbar should remain visible while the user is inside the hero section.
After crossing the hero section, the navbar visibility should depend on scroll direction:

• Scroll DOWN → hide navbar
• Scroll UP → show navbar

This improves user experience and prevents sudden header disappearance at the top of the page.

Fixes #1665

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Tested locally on browser.
• Navbar remains visible inside hero.
• Navbar hides on scroll down after hero.
• Navbar shows on scroll up.
• No console errors.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots or example output

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[harishsehlangia]

This PR is closed due to one edge case that is not covered in this PR. I'll solve that and will raise a new PR.
The edge case is explained now in the comment section of issue #1665