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/SchemaLinkingViewer.test.tsx

@@ -42,7 +42,7 @@ test("(b) clicking toggle switches to table listing a promoted table name", asyn
   // wait for mermaid to load
   await waitFor(() => expect(screen.getByTestId("mm")).toBeInTheDocument());
 
-  const toggle = screen.getByRole("button", { name: /tabella/i });
+  const toggle = screen.getByRole("button", { name: /table/i });
   await userEvent.click(toggle);
 
   expect(screen.getByText("orders")).toBeInTheDocument();
@@ -52,7 +52,7 @@ test("(b) clicking toggle switches to table listing a promoted table name", asyn
 test("(c) oversized linking (>45) shows cap notice", () => {
   render(<SchemaLinkingViewer linking={bigLinking} />);
   expect(
-    screen.getByText(/troppi elementi per il grafico/i)
+    screen.getByText(/too many elements for the graph/i)
   ).toBeInTheDocument();
 });