feat: Add active path highlighting to header navigation (#134)

* feat: Add active path highlighting to header navigation

Implemented visual indication for the current active route in the documentation header navigation.
The active navigation item now displays in orange color to improve user orientation.

- Added usePathname hook to detect current route
- Created isActive helper function to compare paths
- Applied conditional styling based on active state
- Handles language prefix in path comparison

Resolves: SSG-105

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

* adf

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Co-authored-by: Claude <[email protected]>

fix: GitHub README GIF 표시 문제 해결 - repository 내부 파일 사용

fixed #4 docs theme error on first visit (#5)

* fix

* start light