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/viewers/ResultsPanel.test.tsx

@@ -48,8 +48,8 @@ test("(a) renders grid cells for a 2×2 preview result", async () => {
   // Grid must be present, not bold scalar
   expect(screen.queryByTestId("scalar-value")).not.toBeInTheDocument();
 
-  // truncated:true → show the "risultati troncati" indicator
-  expect(screen.getByText(/risultati troncati/i)).toBeInTheDocument();
+  // truncated:true → show the "results truncated" indicator
+  expect(screen.getByText(/results truncated/i)).toBeInTheDocument();
 });
 
 // (b) 1×1 preview → bold scalar shown, no grid
@@ -105,7 +105,7 @@ test("(c) Esporta CSV button calls the export endpoint", async () => {
   // Wait for preview to load (use role to avoid ambiguity with paging panel)
   await waitFor(() => expect(screen.getAllByRole("gridcell")).not.toHaveLength(0));
 
-  const btn = screen.getByRole("button", { name: /esporta csv/i });
+  const btn = screen.getByRole("button", { name: /export csv/i });
   await user.click(btn);
 
   await waitFor(() => expect(exportCalled).toBe(true));
@@ -136,9 +136,9 @@ test("(d) selecting tutti re-fetches preview with the large limit", async () =>
   // Initial fetch uses limit 10
   await waitFor(() => expect(limits).toContain(10));
 
-  // Select "tutti"
-  const select = screen.getByLabelText(/righe da visualizzare/i);
-  await user.selectOptions(select, "tutti");
+  // Select "all"
+  const select = screen.getByLabelText(/rows to display/i);
+  await user.selectOptions(select, "all");
 
   // A re-fetch with the large/unlimited limit must occur
   await waitFor(() => expect(limits.some((l) => (l as number) > 10)).toBe(true));