Splits the AppShell header into:
Row 1 — brand: logo + dynamic context subtitle (route-aware) +
global search + agent boards dropdown
Row 2 — nav: work group (בית · ארכיון) | knowledge group (ספריית
פסיקה · אימון · מתודולוגיה) + admin dropdown (⚙) on the left
Three changes from the previous flat 8-item nav:
1. Grouping reflects intent. Daily-driver pages are in "work", corpus
pages in "knowledge"; system pages (skills · diagnostics · settings)
move into a single ⚙ dropdown so they stop competing for attention.
2. Subtitle is now dynamic. `headerSubtitle(pathname)` resolves the
current section so the user always sees where they are without
scanning the nav row. Case routes show the case number explicitly
("ערר 1234-24" / "ערר 1234-24 · ניסוח").
3. The gold-underline active state is preserved and the admin trigger
inherits it whenever any admin route is active.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
