feat(ui): add toggle button for inbox parent-child nesting

Adds a ListTree icon button in the inbox top bar to toggle nesting
on/off. Preference is persisted in localStorage. When disabled, all
issues display as a flat list without grouping.

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

ui/src/lib/inbox.ts

@@ -7,6 +7,7 @@ export const DISMISSED_KEY = "paperclip:inbox:dismissed";
 export const READ_ITEMS_KEY = "paperclip:inbox:read-items";
 export const INBOX_LAST_TAB_KEY = "paperclip:inbox:last-tab";
 export const INBOX_ISSUE_COLUMNS_KEY = "paperclip:inbox:issue-columns";
+export const INBOX_NESTING_KEY = "paperclip:inbox:nesting";
 export type InboxTab = "mine" | "recent" | "unread" | "all";
 export type InboxApprovalFilter = "all" | "actionable" | "resolved";
 export const inboxIssueColumns = ["status", "id", "assignee", "project", "workspace", "parent", "labels", "updated"] as const;
@@ -151,6 +152,23 @@ export function resolveIssueWorkspaceName(
   return null;
 }
 
+export function loadInboxNesting(): boolean {
+  try {
+    const raw = localStorage.getItem(INBOX_NESTING_KEY);
+    return raw !== "false";
+  } catch {
+    return true;
+  }
+}
+
+export function saveInboxNesting(enabled: boolean) {
+  try {
+    localStorage.setItem(INBOX_NESTING_KEY, String(enabled));
+  } catch {
+    // Ignore localStorage failures.
+  }
+}
+
 export function loadLastInboxTab(): InboxTab {
   try {
     const raw = localStorage.getItem(INBOX_LAST_TAB_KEY);

ui/src/pages/Inbox.tsx

@@ -68,6 +68,7 @@ import {
   RotateCcw,
   UserPlus,
   Search,
+  ListTree,
 } from "lucide-react";
 import { Input } from "@/components/ui/input";
 import { PageTabBar } from "../components/PageTabBar";
@@ -84,10 +85,12 @@ import {
   getRecentTouchedIssues,
   isMineInboxTab,
   loadInboxIssueColumns,
+  loadInboxNesting,
   normalizeInboxIssueColumns,
   resolveIssueWorkspaceName,
   resolveInboxSelectionIndex,
   saveInboxIssueColumns,
+  saveInboxNesting,
   InboxApprovalFilter,
   type InboxIssueColumn,
   saveLastInboxTab,
@@ -110,6 +113,11 @@ type SectionKey =
   | "work_items"
   | "alerts";
 
+/** A flat navigation entry for keyboard j/k traversal that includes expanded children. */
+type NavEntry =
+  | { type: "top"; index: number; item: InboxWorkItem }
+  | { type: "child"; parentIndex: number; issue: Issue };
+
 function firstNonEmptyLine(value: string | null | undefined): string | null {
   if (!value) return null;
   const line = value.split("\n").map((chunk) => chunk.trim()).find(Boolean);
@@ -903,10 +911,20 @@ export function Inbox() {
   ]);
 
   // --- Parent-child nesting for inbox issues ---
+  const [nestingEnabled, setNestingEnabled] = useState(() => loadInboxNesting());
+  const toggleNesting = useCallback(() => {
+    setNestingEnabled((prev) => {
+      const next = !prev;
+      saveInboxNesting(next);
+      return next;
+    });
+  }, []);
   const [collapsedInboxParents, setCollapsedInboxParents] = useState<Set<string>>(new Set());
   const { displayItems: nestedWorkItems, childrenByIssueId } = useMemo(
-    () => buildInboxNesting(filteredWorkItems),
-    [filteredWorkItems],
+    () => nestingEnabled
+      ? buildInboxNesting(filteredWorkItems)
+      : { displayItems: filteredWorkItems, childrenByIssueId: new Map<string, Issue[]>() },
+    [filteredWorkItems, nestingEnabled],
   );
   const toggleInboxParentCollapse = useCallback((parentId: string) => {
     setCollapsedInboxParents((prev) => {
@@ -1429,6 +1447,16 @@ export function Inbox() {
               className="h-8 w-[220px] pl-8 text-xs"
             />
           </div>
+          <Button
+            type="button"
+            variant="outline"
+            size="icon"
+            className={cn("hidden h-8 w-8 shrink-0 sm:inline-flex", nestingEnabled && "bg-accent")}
+            onClick={toggleNesting}
+            title={nestingEnabled ? "Disable parent-child nesting" : "Enable parent-child nesting"}
+          >
+            <ListTree className="h-3.5 w-3.5" />
+          </Button>
           <IssueColumnPicker
             availableColumns={availableIssueColumns}
             visibleColumnSet={visibleIssueColumnSet}