feat(ui): redesign header to two rows with grouped nav (Phase B)

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>

web-ui/src/components/header-context.ts

@@ -0,0 +1,31 @@
+/**
+ * Resolves the dynamic subtitle shown next to the brand in the AppShell
+ * header. Reflects the current section so the user always sees where they
+ * are without scanning the nav row.
+ *
+ * Special-cases case routes (`/cases/{caseNumber}` and `/compose`) so the
+ * subtitle includes the case number — the most useful piece of context
+ * during decision drafting.
+ */
+export function headerSubtitle(pathname: string): string {
+  if (pathname === "/") return "בית";
+
+  if (pathname.startsWith("/cases/")) {
+    const [, , slug] = pathname.split("/");
+    if (!slug || slug === "new") return "תיק חדש";
+    const isCompose = pathname.includes("/compose");
+    const decoded = decodeURIComponent(slug);
+    return isCompose ? `ערר ${decoded} · ניסוח` : `ערר ${decoded}`;
+  }
+
+  if (pathname.startsWith("/archive"))     return "ארכיון";
+  if (pathname.startsWith("/training"))    return "אימון סגנון";
+  if (pathname.startsWith("/precedents"))  return "ספריית פסיקה";
+  if (pathname.startsWith("/methodology")) return "מתודולוגיה";
+  if (pathname.startsWith("/skills"))      return "מיומנויות";
+  if (pathname.startsWith("/diagnostics")) return "אבחון";
+  if (pathname.startsWith("/settings"))    return "הגדרות";
+  if (pathname.startsWith("/feedback"))    return "הערות יו״ר";
+
+  return "ניהול תיקים";
+}