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/widgets/MultiselectWidget.tsx

@@ -32,7 +32,7 @@ export function MultiselectWidget({ descriptor, onRespond }: WidgetProps) {
       {descriptor.title && <p className="font-medium">{descriptor.title}</p>}
       <div className="flex gap-2">
         <button className="text-sm border rounded px-2 py-1" onClick={allChecked ? deselectAll : selectAll}>
-          {allChecked ? "Deseleziona tutti" : "Seleziona tutti"}
+          {allChecked ? "Deselect all" : "Select all"}
         </button>
       </div>
       <div className="flex flex-col gap-1">
@@ -52,7 +52,7 @@ export function MultiselectWidget({ descriptor, onRespond }: WidgetProps) {
         disabled={isDisabled}
         onClick={() => onRespond({ id: descriptor.id, kind: "multiselect", choices: Array.from(checked) })}
       >
-        Conferma
+        Confirm
       </button>
     </div>
   );