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, 142 insertions, 0 deletions

diff --git a/src/features/search-settings/ui/SearchBackendField.svelte b/src/features/search-settings/ui/SearchBackendField.svelte
new file mode 100644
index 0000000..c1ae450
--- /dev/null
+++ b/src/features/search-settings/ui/SearchBackendField.svelte
@@ -0,0 +1,142 @@
+<script lang="ts">
+  /**
+   * The "Search backend" dropdown (Settings pane) — picks the engine backing
+   * the `web_search` tool's SEARCH mode. GLOBAL setting persisted on the
+   * backend (`GET/PUT /search/config`); SAVE ON CHANGE (no Set button).
+   * Unconfigured backends stay listed but disabled; "None (search disabled)"
+   * is the off state. The config affects search mode only — scrape/crawl
+   * always route to the self-hosted Firecrawl backend.
+   *
+   * Thin shell: seeds/reverts the select from the last PERSISTED value, shows
+   * saving/saved/error feedback, and degrades to a disabled field with a
+   * "search not available" hint when the backend's search-config service is
+   * not loaded (503) — matching heartbeat's degrade philosophy.
+   */
+  import { untrack } from "svelte";
+  import {
+    buildSearchBackendOptions,
+    type LoadSearchBackendConfig,
+    NONE_BACKEND_ID,
+    type SaveSearchBackend,
+    searchBackendChanged,
+    searchBackendLabel,
+    type SearchBackendConfig,
+  } from "../logic/view-model";
+
+  let {
+    load,
+    save,
+  }: {
+    /** Fetch the persisted global config (called once on mount). */
+    load: LoadSearchBackendConfig;
+    /** Persist a new backend selection (save-on-change). */
+    save: SaveSearchBackend;
+  } = $props();
+
+  let config = $state<SearchBackendConfig | null>(null);
+  let loading = $state(true);
+  let loadUnavailable = $state(false);
+  let loadError = $state<string | null>(null);
+  let selected = $state("");
+  let lastSeed = $state("");
+  let saving = $state(false);
+  let error = $state<string | null>(null);
+  let justSaved = $state<string | null>(null);
+
+  $effect(() => {
+    // Load ONCE on first mount (the setting is GLOBAL — no workspace
+    // re-seeding); guard against strict-mode double effects via the flag.
+    untrack(async () => {
+      if (config !== null || loadUnavailable) return;
+      loading = true;
+      loadError = null;
+      const result = await load();
+      loading = false;
+      if (result.ok) {
+        config = result.config;
+        selected = result.config.backend;
+        lastSeed = result.config.backend;
+      } else {
+        loadUnavailable = result.unavailable === true;
+        loadError = result.error;
+      }
+    });
+  });
+
+  const options = $derived(config === null ? [] : buildSearchBackendOptions(config));
+  const persisted = $derived(lastSeed);
+
+  async function handleChange(): Promise<void> {
+    if (saving || config === null) return;
+    const next = selected;
+    if (!searchBackendChanged(next, persisted)) return;
+    saving = true;
+    error = null;
+    justSaved = null;
+    const result = await save(next);
+    saving = false;
+    if (result.ok) {
+      config = result.config;
+      // Reflect the persisted value back (the backend is authoritative).
+      selected = result.config.backend;
+      lastSeed = result.config.backend;
+      justSaved = searchBackendLabel(result.config, result.config.backend);
+    } else {
+      error = result.error;
+      // Revert the select to the last persisted value on failure.
+      selected = persisted;
+    }
+  }
+
+  function onSelectChange(): void {
+    error = null;
+    justSaved = null;
+    void handleChange();
+  }
+</script>
+
+<div class="flex flex-col gap-1">
+  <span class="text-xs font-semibold uppercase opacity-60">Search backend</span>
+  {#if loading}
+    <div class="skeleton h-7 w-full" aria-label="Search backend loading"></div>
+  {:else if config === null}
+    <select
+      class="select select-bordered select-sm w-full text-xs"
+      disabled
+      aria-label="Search backend"
+    >
+      <option>{loadUnavailable ? "Search not available" : (loadError ?? "Unavailable")}</option>
+    </select>
+    {#if loadError && !loadUnavailable}
+      <p class="text-xs text-error">{loadError}</p>
+    {/if}
+  {:else}
+    <select
+      class="select select-bordered select-sm w-full text-xs"
+      bind:value={selected}
+      disabled={saving}
+      onchange={onSelectChange}
+      aria-label="Search backend"
+    >
+      {#each options as option (option.id)}
+        <option value={option.id} disabled={option.disabled}>{option.label}</option>
+      {/each}
+    </select>
+    {#if saving}
+      <p class="text-xs opacity-60 flex items-center gap-1">
+        <span class="loading loading-spinner loading-xs"></span>
+        Saving…
+      </p>
+    {:else if error}
+      <p class="text-xs text-error">{error}</p>
+    {:else if justSaved !== null}
+      <p class="text-xs text-success">Saved: {justSaved}.</p>
+    {/if}
+  {/if}
+  {#if !loadUnavailable}
+    <p class="text-xs opacity-50">
+      Affects the web_search tool's <em>search</em> mode only — scrape and crawl always use the
+      self-hosted Firecrawl backend, and the tool's map mode has been removed.
+    </p>
+  {/if}
+</div>