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, 108 insertions, 0 deletions

diff --git a/src/features/search-settings/logic/view-model.test.ts b/src/features/search-settings/logic/view-model.test.ts
new file mode 100644
index 0000000..7f3ceb5
--- /dev/null
+++ b/src/features/search-settings/logic/view-model.test.ts
@@ -0,0 +1,108 @@
+import { describe, expect, it } from "vitest";
+import {
+  buildSearchBackendOptions,
+  NONE_BACKEND_ID,
+  NONE_BACKEND_LABEL,
+  normalizeSearchBackendConfig,
+  type SearchBackendConfig,
+  searchBackendChanged,
+  searchBackendLabel,
+} from "./view-model";
+
+const CONFIG: SearchBackendConfig = {
+  backend: "firecrawl",
+  available: [
+    { id: "firecrawl", label: "Firecrawl (self-hosted)", configured: true },
+    { id: "umans", label: "Umans (Exa)", configured: true },
+  ],
+};
+
+describe("buildSearchBackendOptions", () => {
+  it("lists the available backends in wire order plus the None off state last", () => {
+    expect(buildSearchBackendOptions(CONFIG)).toEqual([
+      { id: "firecrawl", label: "Firecrawl (self-hosted)", disabled: false },
+      { id: "umans", label: "Umans (Exa)", disabled: false },
+      { id: "none", label: NONE_BACKEND_LABEL, disabled: false },
+    ]);
+  });
+
+  it("disables unconfigured backends with a suffix (listed, never an error)", () => {
+    const config: SearchBackendConfig = {
+      backend: "firecrawl",
+      available: [
+        { id: "firecrawl", label: "Firecrawl (self-hosted)", configured: true },
+        { id: "umans", label: "Umans (Exa)", configured: false },
+      ],
+    };
+    const options = buildSearchBackendOptions(config);
+    expect(options[1]).toEqual({
+      id: "umans",
+      label: "Umans (Exa) (not configured)",
+      disabled: true,
+    });
+    expect(options[2]?.id).toBe(NONE_BACKEND_ID);
+  });
+
+  it("prepends an unknown persisted selection so the select can render it", () => {
+    const config: SearchBackendConfig = { backend: "gone", available: [] };
+    const options = buildSearchBackendOptions(config);
+    expect(options[0]).toEqual({ id: "gone", label: "gone (unknown)", disabled: false });
+    expect(options.at(-1)?.id).toBe(NONE_BACKEND_ID);
+  });
+
+  it("does not duplicate the none option when 'none' is selected", () => {
+    const config: SearchBackendConfig = { backend: NONE_BACKEND_ID, available: [] };
+    const options = buildSearchBackendOptions(config);
+    expect(options.filter((o) => o.id === NONE_BACKEND_ID)).toHaveLength(1);
+  });
+});
+
+describe("searchBackendLabel", () => {
+  it("maps a backend id to its advertised label", () => {
+    expect(searchBackendLabel(CONFIG, "umans")).toBe("Umans (Exa)");
+  });
+
+  it("maps 'none' to the off-state label", () => {
+    expect(searchBackendLabel(CONFIG, NONE_BACKEND_ID)).toBe(NONE_BACKEND_LABEL);
+  });
+
+  it("falls back to the raw id for an unknown backend", () => {
+    expect(searchBackendLabel(CONFIG, "gone")).toBe("gone");
+  });
+});
+
+describe("searchBackendChanged", () => {
+  it("detects a real change and a no-op reselect", () => {
+    expect(searchBackendChanged("umans", "firecrawl")).toBe(true);
+    expect(searchBackendChanged("firecrawl", "firecrawl")).toBe(false);
+    expect(searchBackendChanged(NONE_BACKEND_ID, NONE_BACKEND_ID)).toBe(false);
+  });
+});
+
+describe("normalizeSearchBackendConfig", () => {
+  it("normalizes a well-formed response", () => {
+    expect(
+      normalizeSearchBackendConfig({
+        backend: "umans",
+        available: [{ id: "firecrawl", label: "Firecrawl (self-hosted)", configured: true }],
+      }),
+    ).toEqual({
+      backend: "umans",
+      available: [{ id: "firecrawl", label: "Firecrawl (self-hosted)", configured: true }],
+    });
+  });
+
+  it("degrades a malformed/partial response instead of crashing the renderer", () => {
+    expect(normalizeSearchBackendConfig(null)).toEqual({ backend: NONE_BACKEND_ID, available: [] });
+    expect(normalizeSearchBackendConfig({})).toEqual({ backend: NONE_BACKEND_ID, available: [] });
+    expect(
+      normalizeSearchBackendConfig({
+        backend: "firecrawl",
+        available: [null, 42, { id: "" }, { label: "no id" }, { id: "umans" }],
+      }),
+    ).toEqual({
+      backend: "firecrawl",
+      available: [{ id: "umans", label: "umans", configured: false }],
+    });
+  });
+});