fix(frontend): English UI strings for session menu/dialogs; rename empty-guard + cancel test

- SessionMenu: anglicize all Italian labels (Vista divisa → Split view, etc.)
- RenameDialog: add empty-name guard + change title to "Rename session"
- DeleteConfirmDialog: translate title and description to English
- SessionActions.test: add 2 new tests (empty name guard, cancel on DeleteConfirmDialog)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/DeleteConfirmDialog.tsx

@@ -12,9 +12,9 @@ export function DeleteConfirmDialog({ open, label, onOpenChange, onConfirm }: Pr
   return (
     <Dialog open={open} onOpenChange={onOpenChange}>
       <DialogContent>
-        <DialogHeader><DialogTitle>Elimina definitivamente</DialogTitle></DialogHeader>
+        <DialogHeader><DialogTitle>Delete permanently</DialogTitle></DialogHeader>
         <DialogDescription>
-          "{label}" verrà eliminata definitivamente, inclusi tutti i suoi documenti. L'operazione non è reversibile.
+          "{label}" will be permanently deleted, including all of its documents. This action cannot be undone.
         </DialogDescription>
         <DialogFooter>
           <Button type="button" variant="outline" size="sm" onClick={() => onOpenChange(false)}>Cancel</Button>