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, 124 insertions, 0 deletions

diff --git a/src/features/search-settings/logic/view-model.ts b/src/features/search-settings/logic/view-model.ts
new file mode 100644
index 0000000..5d172c9
--- /dev/null
+++ b/src/features/search-settings/logic/view-model.ts
@@ -0,0 +1,124 @@
+/**
+ * Pure core for the search-settings feature — zero DOM, zero effects, zero
+ * Svelte.
+ *
+ * The "Search backend" dropdown (Settings pane) picks the engine backing the
+ * `web_search` tool's SEARCH mode. The selection is GLOBAL and persisted on
+ * the backend (`GET/PUT /search/config`); decided entries are exactly
+ * "Firecrawl (self-hosted)" + "Umans (Exa)" plus "None (search disabled)" as
+ * the off state. Scrape/crawl always route to the self-hosted Firecrawl
+ * backend regardless of the selection; the tool's `map` mode is removed
+ * entirely (backend plan).
+ */
+
+/**
+ * One selectable backend as the backend's wire advertises it (mirrors
+ * `@dispatch/wire`'s `SearchBackendAvailability`).
+ */
+export interface SearchBackendAvailability {
+  readonly id: string;
+  readonly label: string;
+  /** The availability gate (umans: `UMANS_API_KEY` resolves; firecrawl: always). */
+  readonly configured: boolean;
+}
+
+/** The global search-backend config (`GET /search/config` response). */
+export interface SearchBackendConfig {
+  /** The selected backend id, or `"none"` (search off). */
+  readonly backend: string;
+  readonly available: readonly SearchBackendAvailability[];
+}
+
+// ── Injected ports (consumer-defines-port; the composition root adapts the
+//    store's HTTP calls to these shapes). ──────────────────────────────────────
+
+/** Outcome of `GET/PUT /search/config`. `unavailable` marks the 503 degrade
+ * (the backend's search-config service not loaded) so the UI can disable the
+ * field with a "search not available" hint instead of a generic error. */
+export type SearchBackendConfigResult =
+  | { readonly ok: true; readonly config: SearchBackendConfig }
+  | { readonly ok: false; readonly error: string; readonly unavailable?: boolean };
+
+export type LoadSearchBackendConfig = () => Promise<SearchBackendConfigResult>;
+
+export type SaveSearchBackend = (backend: string) => Promise<SearchBackendConfigResult>;
+
+/** The explicit off-state option label. */
+export const NONE_BACKEND_ID = "none";
+export const NONE_BACKEND_LABEL = "None (search disabled)";
+
+/** One rendered `<option>` of the dropdown. */
+export interface SearchBackendOption {
+  readonly id: string;
+  readonly label: string;
+  /** Unconfigured backends stay LISTED but disabled — never an error. */
+  readonly disabled: boolean;
+}
+
+/**
+ * Build the dropdown options from the config: every advertised backend in
+ * wire order (unconfigured ones disabled with a "not configured" suffix),
+ * then the "None (search disabled)" off state appended last. If the persisted
+ * selection is not among them (an unknown id from a newer/older backend), it
+ * is prepended so the select can render it truthfully.
+ */
+export function buildSearchBackendOptions(
+  config: SearchBackendConfig,
+): readonly SearchBackendOption[] {
+  const options: SearchBackendOption[] = config.available.map((b) => ({
+    id: b.id,
+    label: b.configured ? b.label : `${b.label} (not configured)`,
+    disabled: !b.configured,
+  }));
+  if (
+    config.backend !== NONE_BACKEND_ID &&
+    !config.available.some((b) => b.id === config.backend)
+  ) {
+    options.unshift({ id: config.backend, label: `${config.backend} (unknown)`, disabled: false });
+  }
+  options.push({ id: NONE_BACKEND_ID, label: NONE_BACKEND_LABEL, disabled: false });
+  return options;
+}
+
+/** The human-facing label for a backend id (the saved / error messages). */
+export function searchBackendLabel(config: SearchBackendConfig, id: string): string {
+  if (id === NONE_BACKEND_ID) {
+    return NONE_BACKEND_LABEL;
+  }
+  return config.available.find((b) => b.id === id)?.label ?? id;
+}
+
+/** Whether selecting `selected` would change the persisted backend. */
+export function searchBackendChanged(selected: string, persisted: string): boolean {
+  return selected !== persisted;
+}
+
+/**
+ * Normalize the untyped JSON at the network seam so a malformed/partial
+ * response can never crash the renderer (mirrors `normalizeHeartbeatConfig`).
+ * Unknown entries degrade to unconfigured; a missing backend falls back to
+ * `"none"` rendering (the safest default for a UI that must not lie).
+ */
+export function normalizeSearchBackendConfig(raw: unknown): SearchBackendConfig {
+  const obj = (raw ?? {}) as {
+    backend?: unknown;
+    available?: unknown;
+  };
+  const available = Array.isArray(obj.available) ? obj.available : [];
+  return {
+    backend:
+      typeof obj.backend === "string" && obj.backend.length > 0 ? obj.backend : NONE_BACKEND_ID,
+    available: available.flatMap((entry) => {
+      if (entry === null || typeof entry !== "object") return [];
+      const e = entry as { id?: unknown; label?: unknown; configured?: unknown };
+      if (typeof e.id !== "string" || e.id.length === 0) return [];
+      return [
+        {
+          id: e.id,
+          label: typeof e.label === "string" ? e.label : e.id,
+          configured: e.configured === true,
+        },
+      ];
+    }),
+  };
+}