feat(search): add header global search (Phase A) — cases + precedents + docs

Adds an always-visible debounced search input in the AppShell header
that fans out to three independent sources in parallel and renders
per-source result groups with their own loading/empty/error states:

- /api/search/cases (NEW): SQL ILIKE on case_number, address, parties,
  title, subject. Returns small projections, no embeddings needed.
- /api/precedent-library/search (existing): semantic over case-law
  halachot + passages.
- /api/search (existing): semantic over case documents + past decisions.

Cmd/Ctrl+K focuses the input; Esc and click-outside close the panel.
This is Phase A of the header redesign — the bar layout itself is
unchanged; row grouping + dynamic context follow in Phase B.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web-ui/src/components/app-shell.tsx

@@ -13,6 +13,7 @@ import {
   DropdownMenuSeparator,
   DropdownMenuTrigger,
 } from "@/components/ui/dropdown-menu";
+import { GlobalSearch } from "@/components/global-search";
 
 type AgentBoard = {
   prefix: string;
@@ -83,7 +84,7 @@ export function AppShell({ children }: { children: ReactNode }) {
         </Link>
 
         <nav
-          className="me-auto flex items-center gap-1"
+          className="flex items-center gap-1"
           aria-label="ניווט ראשי"
         >
           {NAV_ITEMS.map((item) => {
@@ -114,6 +115,10 @@ export function AppShell({ children }: { children: ReactNode }) {
           })}
         </nav>
 
+        <div className="flex-1 min-w-0 max-w-[460px] mx-4 flex justify-center">
+          <GlobalSearch />
+        </div>
+
         <DropdownMenu>
           <DropdownMenuTrigger
             className="