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/RenameDialog.tsx

@@ -15,6 +15,7 @@ export function RenameDialog({ open, initial, onOpenChange, onSubmit }: Props) {
 
   function submit(e: React.FormEvent) {
     e.preventDefault();
+    if (!name.trim()) return;
     onSubmit(name.trim());
     onOpenChange(false);
   }
@@ -22,7 +23,7 @@ export function RenameDialog({ open, initial, onOpenChange, onSubmit }: Props) {
   return (
     <Dialog open={open} onOpenChange={onOpenChange}>
       <DialogContent>
-        <DialogHeader><DialogTitle>Rinomina sessione</DialogTitle></DialogHeader>
+        <DialogHeader><DialogTitle>Rename session</DialogTitle></DialogHeader>
         <form onSubmit={submit} className="flex flex-col gap-3">
           <div>
             <label className="mb-1.5 block text-xs font-medium text-muted-foreground" htmlFor="rename-name">Name</label>