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.

3 files changed, 75 insertions, 0 deletions

diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index a858927..73b19bc 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -102,6 +102,10 @@ import type {
 import { normalizeNamedAgentRecord, normalizeNamedAgents } from "../features/named-agents";
 import type { PluginsStore } from "../features/plugins";
 import { createPluginsHttp, createPluginsStore } from "../features/plugins";
+import {
+  normalizeSearchBackendConfig,
+  type SearchBackendConfigResult,
+} from "../features/search-settings";
 import type { Tab, TabsState } from "../features/tabs";
 import { createTabsStore, deriveTitle, type TabsStore } from "../features/tabs";
 import {
@@ -531,6 +535,20 @@ export interface AppStore {
    */
   concurrencyStatus(): Promise<ConcurrencyStatusResult>;
   /**
+   * Load the GLOBAL search-backend selection
+   * (`GET /search/config`): which engine backs the `web_search` tool's search
+   * mode. Not workspace-scoped — the selection is global. `unavailable: true`
+   * on the failure variant marks the 503 degrade (the backend's
+   * search-config service not loaded) so the UI can disable the field.
+   */
+  searchBackendConfig(): Promise<SearchBackendConfigResult>;
+  /**
+   * Persist a new search-backend selection
+   * (`PUT /search/config`, body `{ backend }`). `400` (unknown/unconfigured
+   * backend) and `503` (service not loaded) both surface as `ok: false`.
+   */
+  setSearchBackend(backend: string): Promise<SearchBackendConfigResult>;
+  /**
    * Fetch the per-slot release cooldown (ms) for one provider
    * (`GET /concurrency/cooldown/:providerId`). `404` (no concurrency config at
    * all) and `503` (extension not loaded) both surface as `ok: false`.
@@ -2557,6 +2575,56 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
       }
     },
 
+    async searchBackendConfig(): Promise<SearchBackendConfigResult> {
+      try {
+        const res = await fetchImpl(`${httpBase}/search/config`);
+        if (!res.ok) {
+          const errBody = (await res.json().catch(() => null)) as { error?: string } | null;
+          return {
+            ok: false,
+            error: errBody?.error ?? `Search config failed (HTTP ${res.status})`,
+            // The 503 degrade = the backend's search-config service not loaded
+            // (the tool-web-search extension absent) — a distinct UI state.
+            ...(res.status === 503 ? { unavailable: true } : {}),
+          };
+        }
+        // Normalize the untyped JSON at this network seam (pure helper) so a
+        // malformed/partial response can never crash the renderer.
+        const config = normalizeSearchBackendConfig(await res.json());
+        return { ok: true, config };
+      } catch (err) {
+        return {
+          ok: false,
+          error: err instanceof Error ? err.message : "Search config request failed",
+        };
+      }
+    },
+
+    async setSearchBackend(backend: string): Promise<SearchBackendConfigResult> {
+      try {
+        const res = await fetchImpl(`${httpBase}/search/config`, {
+          method: "PUT",
+          headers: { "content-type": "application/json" },
+          body: JSON.stringify({ backend }),
+        });
+        if (!res.ok) {
+          const errBody = (await res.json().catch(() => null)) as { error?: string } | null;
+          return {
+            ok: false,
+            error: errBody?.error ?? `Set search backend failed (HTTP ${res.status})`,
+            ...(res.status === 503 ? { unavailable: true } : {}),
+          };
+        }
+        const config = normalizeSearchBackendConfig(await res.json());
+        return { ok: true, config };
+      } catch (err) {
+        return {
+          ok: false,
+          error: err instanceof Error ? err.message : "Set search backend request failed",
+        };
+      }
+    },
+
     async getConcurrencyCooldown(providerId: string): Promise<ConcurrencyCooldownResult> {
       try {
         const res = await fetchImpl(
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>