M-029 Pass 1 batch 2: migrate 5 two-mutation pages to useTrackedMutation

Drains 10 more useMutation sites (52 -> 42). Each migration declares explicit

invalidates per the M-009 contract.

Pages migrated:

  - DashboardPage.tsx        previewDigest + sendDigest both 'noop' (read-only

                              preview / fire-and-forget email — no client cache impact)

  - DiscoveryPage.tsx        claim + dismiss both invalidate

                              [['discovered-certificates'], ['discovery-summary']]

  - NotificationsPage.tsx    markRead + requeue both invalidate [['notifications']]

  - TargetDetailPage.tsx     update + testConnection both invalidate [['target', id]]

  - TargetsPage.tsx          createTarget + deleteTarget both invalidate [['targets']]

Verification:

  legacy useMutation count   52 -> 42 (-10)

  useTrackedMutation count    4 -> 14 (+10)

Closes 14 of 56 sites toward M-029 Pass 1 completion.

web/src/pages/DashboardPage.tsx

@@ -1,5 +1,6 @@
 import { useState } from 'react';
-import { useQuery, useMutation } from '@tanstack/react-query';
+import { useQuery } from '@tanstack/react-query';
+import { useTrackedMutation } from '../hooks/useTrackedMutation';
 import { useNavigate, useSearchParams } from 'react-router-dom';
 import {
   BarChart, Bar, LineChart, Line, PieChart, Pie, Cell,
@@ -81,15 +82,21 @@ function DigestCard() {
   const [previewHtml, setPreviewHtml] = useState<string | null>(null);
   const [showPreview, setShowPreview] = useState(false);
 
-  const previewMutation = useMutation({
+  const previewMutation = useTrackedMutation({
     mutationFn: previewDigest,
+    invalidates: 'noop',
+    noopReason: 'previewDigest is read-only — server renders HTML; no cached query touched',
     onSuccess: (html) => {
       setPreviewHtml(html);
       setShowPreview(true);
     },
   });
 
-  const sendMutation = useMutation({ mutationFn: sendDigest });
+  const sendMutation = useTrackedMutation({
+    mutationFn: sendDigest,
+    invalidates: 'noop',
+    noopReason: 'sendDigest dispatches an email server-side; no cached client query reflects digest-send state',
+  });
 
   return (
     <>