test: extract buildIssueTree utility and add tests for hierarchy logic

Extract the inline tree-building logic from IssuesList into a pure
`buildIssueTree` function in lib/issue-tree.ts so it can be unit tested.
Add six tests covering: flat lists, parent-child grouping, multi-level
nesting, orphaned sub-tasks promoted to root, empty input, and list
order preservation.

Add two tests to IssueRow.test.tsx covering the new titleSuffix prop:
renders inline after the title when provided, and renders cleanly when
omitted.

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

ui/src/components/IssueRow.test.tsx

@@ -136,4 +136,42 @@ describe("IssueRow", () => {
       root.unmount();
     });
   });
+
+  it("renders titleSuffix inline after the issue title", () => {
+    const root = createRoot(container);
+    const issue = createIssue({ title: "Parent task" });
+
+    act(() => {
+      root.render(
+        <IssueRow
+          issue={issue}
+          titleSuffix={<span data-testid="suffix">(3 sub-tasks)</span>}
+        />,
+      );
+    });
+
+    const titleEl = container.querySelector(".line-clamp-2, .truncate");
+    expect(titleEl?.textContent).toContain("Parent task");
+    expect(titleEl?.textContent).toContain("(3 sub-tasks)");
+    expect(container.querySelector('[data-testid="suffix"]')).not.toBeNull();
+
+    act(() => {
+      root.unmount();
+    });
+  });
+
+  it("renders without error when titleSuffix is omitted", () => {
+    const root = createRoot(container);
+
+    act(() => {
+      root.render(<IssueRow issue={createIssue()} />);
+    });
+
+    const titleEl = container.querySelector(".line-clamp-2, .truncate");
+    expect(titleEl?.textContent).toContain("Inbox item");
+
+    act(() => {
+      root.unmount();
+    });
+  });
 });

ui/src/components/IssuesList.tsx

@@ -23,6 +23,7 @@ import { Checkbox } from "@/components/ui/checkbox";
 import { Collapsible, CollapsibleTrigger, CollapsibleContent } from "@/components/ui/collapsible";
 import { CircleDot, Plus, Filter, ArrowUpDown, Layers, Check, X, ChevronRight, List, Columns3, User, Search } from "lucide-react";
 import { KanbanBoard } from "./KanbanBoard";
+import { buildIssueTree } from "../lib/issue-tree";
 import type { Issue } from "@paperclipai/shared";
 
 /* ── Helpers ── */
@@ -667,16 +668,7 @@ export function IssuesList({
             )}
             <CollapsibleContent>
               {(() => {
-                const itemIds = new Set(group.items.map((i) => i.id));
-                const roots = group.items.filter((i) => !i.parentId || !itemIds.has(i.parentId));
-                const childMap = new Map<string, Issue[]>();
-                for (const item of group.items) {
-                  if (item.parentId && itemIds.has(item.parentId)) {
-                    const arr = childMap.get(item.parentId) ?? [];
-                    arr.push(item);
-                    childMap.set(item.parentId, arr);
-                  }
-                }
+                const { roots, childMap } = buildIssueTree(group.items);
 
                 const renderIssueRow = (issue: Issue, depth: number) => {
                   const children = childMap.get(issue.id) ?? [];