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/NavSessions.tsx

@@ -26,7 +26,7 @@ export function NavSessions({ activeSessionId, onSessionSelect }: Props) {
   if (sessions.length === 0) {
     return (
       <p className="px-1 py-6 text-center text-xs text-muted-foreground">
-        Nessuna sessione. Inizia con <span className="font-medium text-foreground">Nuova domanda</span>.
+        No sessions yet. Start with <span className="font-medium text-foreground">New question</span>.
       </p>
     );
   }