feat(acks): surface 999 rejections via sidebar badge + table sort

The 1,156 accepted acks were burying the 5 rejections — the operator
had to scroll the whole table to find them. Two changes:

1. Sidebar '999 ACKs' nav item gets a warning-toned badge showing
   the total rejected segment count from aggregates.rejected_count
   (mirrors the Reconciliation unmatched badge pattern). One number,
   visible from any page.

2. Acks page table now sorts rejected rows to the top, then
   newest-id-first. All 5 rejections fit on page 1 since they're a
   tiny fraction of the total — no pagination needed.

New files:
- src/hooks/useAckStats.ts (lightweight aggregates-only fetch)
- src/hooks/useAckStats.test.tsx (2 tests)
- src/components/Sidebar.test.tsx (3 tests)

Pre-existing baseline: 10 frontend failures (api.test.ts exportBatch837,
tail-stream.test.ts acks/ta1_acks targeting, Inbox/InboxHeader copy).
Unchanged.

src/pages/Acks.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useState } from "react";
+import { useCallback, useMemo, useState } from "react";
 import { CheckCircle2, Download, Mail, ShieldCheck } from "lucide-react";
 import {
   Table,
@@ -63,7 +63,21 @@ export function Acks() {
   // dedupes by id (first write wins), so a snapshot replay on
   // reconnect can't double-count an existing row.
   const mergedItems = useMergedTail("acks", data?.items ?? []);
-  const items = mergedItems;
+  // Display order: rejected rows float to the top, then newest-first
+  // by id. The server returns id-DESC so accepted rows already
+  // follow that order — we only need to bubble the rejections up
+  // (the operator flagged on 2026-07-02 that 5 rejections were
+  // buried inside 1,156 accepted acks and hard to find). All 5 fit
+  // on page 1 since they're a tiny fraction of the total — the
+  // operator never needs to paginate to find them.
+  const items = useMemo(() => {
+    return [...mergedItems].sort((a, b) => {
+      const ra = a.rejectedCount > 0 ? 1 : 0;
+      const rb = b.rejectedCount > 0 ? 1 : 0;
+      if (ra !== rb) return rb - ra; // rejected first
+      return b.id - a.id;             // newest id first within each group
+    });
+  }, [mergedItems]);
   const totalCount = data?.total ?? 0;
   // Server-side aggregates — sum over the *full* ACK set, not just the
   // visible page. Without this, the KPI strip silently under-reports