fix: count all descendants in collapsed badge and prune stale localStorage IDs

Address two Greptile review comments:

1. Collapsed parent badge now shows total descendant count at all depths
   rather than direct-child count only. Add `countDescendants` utility to
   issue-tree.ts (recursive, uses existing childMap) and replace
   `children.length` with it in the titleSuffix badge.

2. Add a useEffect that prunes stale IDs from `collapsedParents` whenever
   the issues prop changes. Deleted or reassigned issues previously left
   orphan IDs in localStorage indefinitely; the effect filters to only IDs
   that appear as a parentId in the current issue list and persists the
   cleaned array via updateView.

Add four unit tests for countDescendants: leaf node, single-level,
multi-level, and unknown ID.

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

ui/src/lib/issue-tree.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 import type { Issue } from "@paperclipai/shared";
-import { buildIssueTree } from "./issue-tree";
+import { buildIssueTree, countDescendants } from "./issue-tree";
 
 function makeIssue(id: string, parentId: string | null = null): Issue {
   return {
@@ -96,3 +96,35 @@ describe("buildIssueTree", () => {
     expect(childMap.get("p1")?.map((c) => c.id)).toEqual(["c1", "c2"]);
   });
 });
+
+describe("countDescendants", () => {
+  it("returns 0 for a leaf node", () => {
+    const { childMap } = buildIssueTree([makeIssue("a")]);
+    expect(countDescendants("a", childMap)).toBe(0);
+  });
+
+  it("returns direct child count for a single-level parent", () => {
+    const { childMap } = buildIssueTree([
+      makeIssue("p"),
+      makeIssue("c1", "p"),
+      makeIssue("c2", "p"),
+    ]);
+    expect(countDescendants("p", childMap)).toBe(2);
+  });
+
+  it("counts all descendants across multiple levels", () => {
+    // P → C → G1, G2  (P has 3 total descendants: C, G1, G2)
+    const { childMap } = buildIssueTree([
+      makeIssue("p"),
+      makeIssue("c", "p"),
+      makeIssue("g1", "c"),
+      makeIssue("g2", "c"),
+    ]);
+    expect(countDescendants("p", childMap)).toBe(3);
+  });
+
+  it("returns 0 for an id not in the childMap", () => {
+    const { childMap } = buildIssueTree([makeIssue("a"), makeIssue("b")]);
+    expect(countDescendants("nonexistent", childMap)).toBe(0);
+  });
+});

ui/src/lib/issue-tree.ts

@@ -25,3 +25,12 @@ export function buildIssueTree(items: Issue[]): IssueTree {
   }
   return { roots, childMap };
 }
+
+/**
+ * Returns the total number of descendants (all depths) of `id` in `childMap`.
+ * Used to accurately label collapsed parent badges like "(3 sub-tasks)".
+ */
+export function countDescendants(id: string, childMap: Map<string, Issue[]>): number {
+  const children = childMap.get(id) ?? [];
+  return children.reduce((sum, c) => sum + 1 + countDescendants(c.id, childMap), 0);
+}