Hardening shows who still uses legacy mail apps (LP-15)

The impact panel, above the statement, shown before anything can change:
"3 accounts used a legacy mail app in the last 30 days. Their mail apps
will stop working the moment you turn this on:", then one line per
account with every protocol it used and how long ago it last did, most
recent first. With nobody, it says so in one line. While the switch is
off it isn't shown -- there is nothing left to warn about.

It reads recentLegacyUse from inbuxa:ProtocolPolicy, which the server
fills from one timestamp per account and protocol (inbuxa-server
feat/legacy-use-panel). A server without it gets no panel at all rather
than a false "nobody": null and an empty list are kept apart.

Protocols read as the table names them (IMAP, POP3, ManageSieve, SMTP
submission), and "2 days ago" comes from Intl.RelativeTimeFormat in the
reader's language.

src/features/hardening/LegacyProtocolsPage.tsx

@@ -25,8 +25,10 @@ import { useAccountStore } from '@/stores/accountStore';
 import { toast } from '@/hooks/use-toast';
 import { cn } from '@/lib/utils';
 import {
+  ago,
   CONFIRM_PHRASE,
   describeListener,
+  impactEntries,
   fetchProtocolPolicy,
   phraseMatches,
   PolicyUnavailable,
@@ -35,6 +37,7 @@ import {
   type PolicyListener,
   type ProtocolPolicy,
   type ProtocolRow,
+  type RecentUse,
 } from './protocolPolicy';
 
 type Load = { kind: 'loading' } | { kind: 'ready'; policy: ProtocolPolicy } | { kind: 'error'; message: string };
@@ -156,6 +159,8 @@ export function LegacyProtocolsPage() {
 
       <ProtocolTable rows={protocolRows(policy, listeners)} off={off} />
 
+      {!off && policy.recentLegacyUse && <ImpactPanel recent={policy.recentLegacyUse} />}
+
       {(off || confirming) && <Statement listeners={listeners} />}
 
       {!off && canUpdate && !confirming && (
@@ -316,6 +321,48 @@ function ProtocolTable({ rows, off }: { rows: ProtocolRow[]; off: boolean }) {
   );
 }
 
+/**
+ * The impact panel (LP-15): who would notice, shown before anything can
+ * change. With nobody, it says so in one line.
+ */
+function ImpactPanel({ recent }: { recent: RecentUse[] }) {
+  const { t, i18n } = useTranslation();
+  const entries = impactEntries(recent);
+  // Read once, when the panel appears: "2 days ago" needn't tick.
+  const [now] = useState(() => Date.now());
+  if (entries.length === 0) {
+    return (
+      <p className="rounded-xl border px-4 py-3 text-sm text-muted-foreground">
+        {t('legacyProtocols.impactNone', 'No account used a legacy mail app in the last 30 days.')}
+      </p>
+    );
+  }
+  return (
+    <section className="space-y-2 rounded-xl border p-4 text-sm">
+      <p>
+        <strong>
+          {t('legacyProtocols.impactCount', {
+            count: entries.length,
+            defaultValue_one: '1 account used a legacy mail app in the last 30 days.',
+            defaultValue_other: '{{count}} accounts used a legacy mail app in the last 30 days.',
+          })}
+        </strong>{' '}
+        {t('legacyProtocols.impactLead', 'Their mail apps will stop working the moment you turn this on:')}
+      </p>
+      <ul className="max-h-72 space-y-1 overflow-y-auto">
+        {entries.map((entry) => (
+          <li key={entry.name} className="flex flex-wrap gap-x-2">
+            <span className="font-medium">{entry.name}</span>
+            <span className="text-muted-foreground">
+              {entry.protocols.join(', ')} · {ago(entry.lastUsedAt, now, i18n.language)}
+            </span>
+          </li>
+        ))}
+      </ul>
+    </section>
+  );
+}
+
 /** The statement (LP-16), at server scope, with the firewall note (LP-20). */
 function Statement({ listeners }: { listeners: PolicyListener[] }) {
   const { t } = useTranslation();