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/lib/issue-tree.ts

@@ -0,0 +1,27 @@
+import type { Issue } from "@paperclipai/shared";
+
+export interface IssueTree {
+  roots: Issue[];
+  childMap: Map<string, Issue[]>;
+}
+
+/**
+ * Builds a parent→children tree from a flat list of issues.
+ *
+ * - `roots` contains issues whose parent is absent from the list (or have no
+ *   parent at all), so orphaned sub-tasks are always visible at root level.
+ * - `childMap` maps each parent id to its direct children in list order.
+ */
+export function buildIssueTree(items: Issue[]): IssueTree {
+  const itemIds = new Set(items.map((i) => i.id));
+  const roots = items.filter((i) => !i.parentId || !itemIds.has(i.parentId));
+  const childMap = new Map<string, Issue[]>();
+  for (const item of items) {
+    if (item.parentId && itemIds.has(item.parentId)) {
+      const arr = childMap.get(item.parentId) ?? [];
+      arr.push(item);
+      childMap.set(item.parentId, arr);
+    }
+  }
+  return { roots, childMap };
+}