Remove redundant CSS properties from hidden elements

[Copilot]

Description

Removed redundant CSS declarations on elements using display: none. When an element has display: none, it's removed from the document flow entirely—additional properties like visibility: hidden, height: 0, margin: 0, padding: 0 have no effect and clutter the stylesheet.

Changes:

• Simplified .downloads and header selectors to use only display: none !important
• Removed all child element styles for hidden parents (children inherit invisibility)
• Cleaned up redundant media query overrides

Impact:

• 44 lines removed
• Improved maintainability
• No functional changes

Related Issue

Addresses feedback from #140

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor (code change that neither fixes a bug nor adds a feature)

Checklist

• I have added tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have updated the CHANGELOG
• My changes do not introduce breaking changes, or breaking changes are documented

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