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/web/static/site.css

@@ -882,6 +882,15 @@ samp {
 	color: var(--text-dim);
 }
 
+.search-areas {
+	margin: 0.6rem 0 0;
+	padding: 0;
+	list-style: none;
+	display: flex;
+	flex-wrap: wrap;
+	gap: 0.4rem 1.1rem;
+}
+
 .results {
 	margin: 0;
 	padding: 0;