The skill browser sidebar now uses categorized expandable domains

The browse sidebar now groups domains into expandable categories and keeps one tree open at a time.

The skill browser sidebar now follows the platform navigation pattern more closely.

Domains are grouped under categories such as Software Engineering.
Only one category tree expands at a time.
Sidebar color, type size, active states, and mini cards were tuned to match the Upsoma visual system.

This makes browsing domains feel more structured without making the main view heavier.