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.

2 files changed, 7 insertions, 0 deletions

diff --git a/src/app/views/ExtensionsPane.svelte b/src/app/views/ExtensionsPane.svelte
index 7a2343e..6afc2bf 100644
--- a/src/app/views/ExtensionsPane.svelte
+++ b/src/app/views/ExtensionsPane.svelte
@@ -17,6 +17,7 @@
   import { manifest as markdownManifest } from "../../features/markdown";
   import { manifest as mcpManifest } from "../../features/mcp";
   import { manifest as namedAgentsManifest } from "../../features/named-agents";
+  import { manifest as searchSettingsManifest } from "../../features/search-settings";
   import { manifest as settingsManifest } from "../../features/settings";
   import { manifest as smartScrollManifest } from "../../features/smart-scroll";
   import { SurfaceView } from "../../features/surface-host";
@@ -60,6 +61,7 @@
     concurrencyManifest,
     visionManifest,
     namedAgentsManifest,
+    searchSettingsManifest,
   ].map((m) => [m.name, m.description] as const);
 </script>
 
diff --git a/src/app/views/SettingsPane.svelte b/src/app/views/SettingsPane.svelte
index 67a88ec..f119103 100644
--- a/src/app/views/SettingsPane.svelte
+++ b/src/app/views/SettingsPane.svelte
@@ -8,6 +8,7 @@
    */
   import { tick } from "svelte";
   import { ChatLimitField, type ChatLimitSaveResult } from "../../features/settings";
+  import { SearchBackendField } from "../../features/search-settings";
   import type { AppViewPorts } from "../view-ports";
 
   let { ports }: { ports: AppViewPorts } = $props();
@@ -31,4 +32,8 @@
 
 <div class="flex flex-col gap-3">
   <ChatLimitField chatLimit={store.chatLimit} save={saveChatLimit} />
+  <SearchBackendField
+    load={() => store.searchBackendConfig()}
+    save={(backend) => store.setSearchBackend(backend)}
+  />
 </div>