Remove sticky header positioning and Issue #127 references from theme.css

[Copilot]

Description

Removes obsolete sticky header CSS rules and Issue #127 comments from theme.css. Since scroll-margin-top properties were removed from headings (no anchor offset needed), the sticky positioning implementation is no longer necessary.

Changes:

• Removed position: sticky, top: 0, z-index: 100 from header styles
• Removed Issue [Feature]: Sticky/Smaller Header in GitHub Pages #127 comments (lines 453, 696)
• Removed scroll-related comment references

Header now renders as static banner instead of sticky-positioned element.

Related Issue

Addresses review 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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