feat(frontend): English UI + 'Datamart Builder with Human In The Loop' tagline

The interface is now fully English regardless of the chat language (session
titles stay in whatever language the user typed). Changed the right-rail
subtitle to 'Datamart Builder with Human In The Loop'. Translated all chrome:
new-question/settings dialogs, workflow phase strip (F1-F8), session rail,
steer input, widgets (select/multiselect/freetext/fallback/reserved controls)
and viewers (SQL, results, schema linking). Test matchers updated in lockstep.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/SettingsDialog.test.tsx

@@ -54,7 +54,7 @@ test("provider change filters the model list", async () => {
 
   const provider = (await screen.findByLabelText(/provider/i)) as HTMLSelectElement;
   await userEvent.selectOptions(provider, "anthropic");
-  const model = screen.getByLabelText(/modello/i) as HTMLSelectElement;
+  const model = screen.getByLabelText(/model/i) as HTMLSelectElement;
   const optionValues = Array.from(model.options).map((o) => o.value);
   expect(optionValues).toContain("claude-opus-4-8");
   expect(optionValues).not.toContain("glm-5.2");
@@ -65,14 +65,14 @@ test("Salva PUTs the selected settings", async () => {
   server.use(
     http.put("http://localhost:8787/settings", async ({ request }) => {
       body = await request.json();
-      return HttpResponse.json(body);
+      return HttpResponse.json({ workspace: "psd", provider: "zai", model: "glm-5.2", thinking: "high" });
     }),
   );
   renderDialog();
   await userEvent.click(screen.getByRole("button", { name: /settings/i }));
   await screen.findByLabelText(/workspace/i);
   await userEvent.selectOptions(screen.getByLabelText(/thinking/i), "high");
-  await userEvent.click(screen.getByRole("button", { name: /salva/i }));
+  await userEvent.click(screen.getByRole("button", { name: /save/i }));
 
   await waitFor(() =>
     expect(body).toEqual({ workspace: "psd", provider: "zai", model: "glm-5.2", thinking: "high" }),