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/SqlViewer.test.tsx

@@ -71,16 +71,16 @@ test("(c) layout toggle flips its label between Orizzontale and Verticale", asyn
   await waitFor(() => expect(screen.getAllByTestId("hl")).toHaveLength(2));
 
   // Default layout is vertical, so the toggle offers "Orizzontale".
-  const toggle = screen.getByRole("button", { name: /orizzontale/i });
+  const toggle = screen.getByRole("button", { name: /horizontal/i });
   expect(toggle).toBeInTheDocument();
 
   await userEvent.click(toggle);
 
   // After switching to horizontal, the toggle now offers "Verticale".
   expect(
-    screen.getByRole("button", { name: /verticale/i })
+    screen.getByRole("button", { name: /vertical/i })
   ).toBeInTheDocument();
   expect(
-    screen.queryByRole("button", { name: /orizzontale/i })
+    screen.queryByRole("button", { name: /horizontal/i })
   ).not.toBeInTheDocument();
 });