feat: show sub-task count in title when parent is collapsed

When a parent issue is collapsed, its title is suffixed with
"(N sub-tasks)" so the count remains visible at a glance.
The suffix disappears when the parent is expanded.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/IssueRow.tsx

@@ -18,6 +18,7 @@ interface IssueRowProps {
   mobileMeta?: ReactNode;
   desktopTrailing?: ReactNode;
   trailingMeta?: ReactNode;
+  titleSuffix?: ReactNode;
   unreadState?: UnreadState | null;
   onMarkRead?: () => void;
   onArchive?: () => void;
@@ -35,6 +36,7 @@ export function IssueRow({
   mobileMeta,
   desktopTrailing,
   trailingMeta,
+  titleSuffix,
   unreadState = null,
   onMarkRead,
   onArchive,
@@ -63,7 +65,7 @@ export function IssueRow({
       </span>
       <span className="flex min-w-0 flex-1 flex-col gap-1 sm:contents">
         <span className="line-clamp-2 text-sm sm:order-2 sm:min-w-0 sm:flex-1 sm:truncate sm:line-clamp-none">
-          {issue.title}
+          {issue.title}{titleSuffix}
         </span>
         <span className="flex items-center gap-2 sm:order-1 sm:shrink-0">
           {desktopLeadingSpacer ? (

ui/src/components/IssuesList.tsx

@@ -697,6 +697,11 @@ export function IssuesList({
                         issue={issue}
                         issueLinkState={issueLinkState}
                         className={isChild ? "pl-6 sm:pl-7" : undefined}
+                        titleSuffix={hasChildren && !isExpanded ? (
+                          <span className="ml-1.5 text-xs text-muted-foreground">
+                            ({children.length} sub-task{children.length !== 1 ? "s" : ""})
+                          </span>
+                        ) : undefined}
                         mobileLeading={
                           hasChildren ? (
                             <button type="button" onClick={toggleCollapse}>