feat(search-backend-selection): Search backend dropdown in Settings

- New features/search-settings: pure view-model (options = available backends
  ∪ 'None (search disabled)', unconfigured entries disabled with a suffix,
  unknown persisted selection prepended, network-seam normalization) + the
  SearchBackendField dropdown (save-on-change, spinner/saved/error states,
  revert-to-persisted on failure, 503 -> disabled field with a 'search not
  available' hint, loading skeleton, help text noting search-mode-only scope).
- store.svelte.ts: searchBackendConfig() / setSearchBackend() over the global
  GET/PUT /search/config routes (503 marked unavailable on the failure variant).
- SettingsPane renders the field under the chat limit; ExtensionsPane lists
  the new module manifest.
- .dispatch mirrors: wire + transport-contract deltas for the new types/routes.

1 files changed, 22 insertions, 0 deletions

diff --git a/src/features/search-settings/index.ts b/src/features/search-settings/index.ts
new file mode 100644
index 0000000..a6ea160
--- /dev/null
+++ b/src/features/search-settings/index.ts
@@ -0,0 +1,22 @@
+export type {
+  LoadSearchBackendConfig,
+  SaveSearchBackend,
+  SearchBackendAvailability,
+  SearchBackendConfig,
+  SearchBackendConfigResult,
+} from "./logic/view-model";
+export {
+  buildSearchBackendOptions,
+  NONE_BACKEND_ID,
+  NONE_BACKEND_LABEL,
+  normalizeSearchBackendConfig,
+  searchBackendChanged,
+  searchBackendLabel,
+} from "./logic/view-model";
+export { default as SearchBackendField } from "./ui/SearchBackendField.svelte";
+
+/** Public module manifest — aggregated by the shell's "Loaded Modules" view. */
+export const manifest = {
+  name: "Search Settings",
+  description: "Global search backend selection for the web_search tool (GET/PUT /search/config)",
+} as const;