feat(ui): nest parent-child issues in inbox/mine view

Groups child issues under their parent in the inbox, matching the
nesting pattern used on the issues list page. Parent groups sort by
the most recent activity across all family members. Archived parents
don't hide their children — orphans show independently.

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

ui/src/lib/inbox.ts

@@ -314,6 +314,68 @@ export function getInboxWorkItems({
   });
 }
 
+/**
+ * Groups parent-child issues in a flat InboxWorkItem list.
+ *
+ * - Children whose parent is also in the list are removed from the top level
+ *   and stored in `childrenByIssueId`.
+ * - The parent's sort timestamp becomes max(parent, children) so that a group
+ *   with a recently-updated child floats to the top.
+ * - If a parent is absent (e.g. archived), children remain as independent roots.
+ */
+export function buildInboxNesting(items: InboxWorkItem[]): {
+  displayItems: InboxWorkItem[];
+  childrenByIssueId: Map<string, Issue[]>;
+} {
+  const issueItems: (InboxWorkItem & { kind: "issue" })[] = [];
+  const nonIssueItems: InboxWorkItem[] = [];
+  for (const item of items) {
+    if (item.kind === "issue") issueItems.push(item as InboxWorkItem & { kind: "issue" });
+    else nonIssueItems.push(item);
+  }
+
+  const issueIdSet = new Set(issueItems.map((i) => i.issue.id));
+  const childrenByIssueId = new Map<string, Issue[]>();
+  const childIds = new Set<string>();
+
+  for (const item of issueItems) {
+    const { issue } = item;
+    if (issue.parentId && issueIdSet.has(issue.parentId)) {
+      childIds.add(issue.id);
+      const arr = childrenByIssueId.get(issue.parentId) ?? [];
+      arr.push(issue);
+      childrenByIssueId.set(issue.parentId, arr);
+    }
+  }
+
+  // Sort each child list by most recent activity
+  for (const children of childrenByIssueId.values()) {
+    children.sort(sortIssuesByMostRecentActivity);
+  }
+
+  // Build root issue items with group-adjusted timestamps
+  const rootIssueItems: InboxWorkItem[] = issueItems
+    .filter((item) => !childIds.has(item.issue.id))
+    .map((item) => {
+      const children = childrenByIssueId.get(item.issue.id);
+      if (!children?.length) return item;
+      const maxChildTs = Math.max(...children.map(issueLastActivityTimestamp));
+      return { ...item, timestamp: Math.max(item.timestamp, maxChildTs) };
+    });
+
+  // Merge and re-sort
+  const displayItems = [...rootIssueItems, ...nonIssueItems].sort((a, b) => {
+    const diff = b.timestamp - a.timestamp;
+    if (diff !== 0) return diff;
+    if (a.kind === "issue" && b.kind === "issue") {
+      return sortIssuesByMostRecentActivity(a.issue, b.issue);
+    }
+    return 0;
+  });
+
+  return { displayItems, childrenByIssueId };
+}
+
 export function shouldShowInboxSection({
   tab,
   hasItems,