Signals in a folder of their own in the feed list, as an OPML's feeds are (#255)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/dialogs.ts

@@ -797,7 +797,7 @@ $('#feedlist').onkeydown=(ev: KeyboardEvent)=>{
   const row=(ev.target as Element).closest<HTMLElement>('[data-id]'); if(!row) return;
   const id=row.dataset.id!;
   if((ev.key==='Enter'||ev.key===' ')&&ev.target===row) row.click();
-  else if(row.classList.contains('group')&&
+  else if(row.matches('.group,.sigs')&&
     (ev.key==='ArrowRight'&&!expanded.has(id)||ev.key==='ArrowLeft'&&expanded.has(id))) toggleGroup(id);
   else return;
   ev.preventDefault(); ev.stopPropagation();