Site-wide rich search with keyword highlighting + header search box

- /Search: searches shifts, hiring openings, and applicants together via
  Postgres ILIKE (every term must match across role/city/facility/title/
  description/tags/person). Results grouped per type.
- Keyword highlighting (<mark>) extended to shift & job cards (was talent-only),
  so matches stand out everywhere.
- Persistent header search box (.nav-search) → /Search; big hero box on the
  page itself.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/JobsMedical.Web/Pages/Shared/_Layout.cshtml

@@ -117,6 +117,10 @@
                     <a asp-page="/Facilities/Index" class="@(path.StartsWith("/Facilities") ? "active" : null)">مراکز درمانی</a>
                     <a asp-page="/Calendar/Index" class="@(path.StartsWith("/Calendar") ? "active" : null)">تقویم هفتگی</a>
                 </nav>
+                <form class="nav-search" method="get" action="/Search" role="search">
+                    <input type="search" name="Q" placeholder="جستجو…" aria-label="جستجو" />
+                    <button type="submit" aria-label="جستجو">🔎</button>
+                </form>
                 <div class="header-actions">
                     <a class="btn btn-accent btn-sm cta-post" asp-page="/Employer/Index" data-tour="post">＋ ثبت آگهی</a>
                     @if (User.Identity?.IsAuthenticated == true)