search: make the navigation form work from every page (AUD-151)

The nav form carried hx-get="/search" hx-target="#search-results", but
#search-results exists only on /search. On every other page htmx found no
target and silently discarded the response: a search box that looks like it
works and does nothing.

- SearchForm is now a plain GET with no htmx attributes (navigation and the
  landing page), so it navigates to /search?q=… from any page and with
  JavaScript disabled.
- SearchFormLive keeps the in-place swap on the search page, where the target
  does exist. For HX-Request the server now returns the block's *contents*
  (SearchResultsBody), so the fragment no longer nests a second element with
  id="search-results" inside the live region.
- Empty query shows a prompt instead of an empty list. A zero-match query says
  "Nothing found" and offers the six areas plus the evidence index.
- The result count is role="status" inside the existing aria-live container, so
  a screen reader reads "4 results for …" rather than a run of titles.

Tests (internal/server, internal/site):
- the nav form carries no htmx attribute on nine pages, and no built page
  carries an hx-target whose element is missing (walks all of dist/);
- search submitted from /, /edge/ and /platform/ lands on a page with results
  with no HX-Request header, i.e. the JavaScript-disabled path;
- empty query, zero match and the announced count;
- a content change followed by a build is visible in dist/search.db. That is
  the deploy path: deploy/deploy.sh runs `make dist` before it stages a
  release, and search.Build removes and rewrites the index every build.

DoD: search from home, /edge and /platform lands on results; works with
JavaScript disabled; zero-match query shows a message; content change followed
by a deploy is reflected in search; no htmx request targets a missing element.

(cherry picked from commit a56c795768e1e9b10204aae04c3612d7a96775a7)

internal/site/site.go

@@ -66,6 +66,16 @@ func (Shell) Nav(area string) []templates.NavItem {
 	return items
 }
 
+// AreaLinks is the six areas as plain links. The search page offers them when a
+// query matches nothing, so an empty result is still a way into the site.
+func (Shell) AreaLinks() []templates.LinkView {
+	out := make([]templates.LinkView, 0, len(content.Areas))
+	for _, a := range content.Areas {
+		out = append(out, templates.LinkView{Label: a.Title, Href: "/" + a.Key + "/"})
+	}
+	return out
+}
+
 // RouteBinding is the Mode A driver: one pre-rendered fragment per click.
 func (Shell) RouteBinding() stage.Binding {
 	return stage.RouteDriver{BaseURL: "/stage"}.Bind()
@@ -114,6 +124,7 @@ func (sh Shell) SearchView(indexed int, q string, results []search.Result, faile
 		Results: results,
 		Indexed: indexed,
 		Failed:  failed,
+		Areas:   sh.AreaLinks(),
 	}
 }