* feat: group navigation into Keywords, Domain, and AI dropdowns
Reorganize the top nav from 7 flat items into grouped dropdowns:
- Keywords: Keyword Research, Saved Keywords, Rank Tracking
- Domain: Domain Overview, Backlinks, Site Audit
- AI: standalone link
Mobile sidebar uses section headers for the same groupings.
Active dropdown items are highlighted with primary tint.
* fix: use matchSegment lookup instead of hardcoded array indices in nav groups
e68ced7203