feat(ui/alerts): SilencesPage prefills Rule ID from ?ruleId= query param

Used by InboxPage's 'Silence rule… → Custom…' flow to carry the alert's
ruleId into the silence creation form.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/Alerts/SilencesPage.tsx

@@ -1,4 +1,5 @@
-import { useState } from 'react';
+import { useState, useEffect } from 'react';
+import { useSearchParams } from 'react-router';
 import { BellOff } from 'lucide-react';
 import {
   Button, FormField, Input, useToast, DataTable,
@@ -28,6 +29,12 @@ export default function SilencesPage() {
   const [hours, setHours] = useState(1);
   const [pendingEnd, setPendingEnd] = useState<AlertSilenceResponse | null>(null);
 
+  const [searchParams] = useSearchParams();
+  useEffect(() => {
+    const r = searchParams.get('ruleId');
+    if (r) setMatcherRuleId(r);
+  }, [searchParams]);
+
   if (isLoading) return <PageLoader />;
   if (error) return <div className={css.page}>Failed to load silences: {String(error)}</div>;