ui(alerts): Silences page adopts Rules UX — top-right button + modal form

Before: the Silences page rendered an always-visible 4-field form strip
above the list, taking room even when the environment had zero silences.
Inconsistent with Rules, which puts a "New rule" action in the page
header and reserves the content area for either the list or an empty
state.

After: header mirrors Rules — title + subtitle on the left, a "New
silence" primary button on the right. The create form moved into a
Modal opened by that button (and by the empty-state's "Create silence"
action). `?ruleId=` deep links still work: the param is read on mount,
prefills the Rule ID field, and auto-opens the modal — preserving the
InboxPage "Silence rule… → Custom…" flow.

Dropped: unused `sectionStyles` import.

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

ui/src/pages/Alerts/SilencesPage.tsx

@@ -3,7 +3,7 @@ import { useSearchParams } from 'react-router';
 import { BellOff } from 'lucide-react';
 import {
   Button, FormField, Input, useToast, DataTable,
-  EmptyState, ConfirmDialog, MonoText,
+  EmptyState, ConfirmDialog, MonoText, Modal,
 } from '@cameleer/design-system';
 import type { Column } from '@cameleer/design-system';
 import { PageLoader } from '../../components/PageLoader';
@@ -14,7 +14,6 @@ import {
   type AlertSilenceResponse,
 } from '../../api/queries/alertSilences';
 import { describeApiError } from '../../api/errors';
-import sectionStyles from '../../styles/section-card.module.css';
 import tableStyles from '../../styles/table-section.module.css';
 import css from './alerts-page.module.css';
 
@@ -29,13 +28,29 @@ export default function SilencesPage() {
   const [matcherAppSlug, setMatcherAppSlug] = useState('');
   const [hours, setHours] = useState(1);
   const [pendingEnd, setPendingEnd] = useState<AlertSilenceResponse | null>(null);
+  const [createOpen, setCreateOpen] = useState(false);
 
   const [searchParams] = useSearchParams();
   useEffect(() => {
     const r = searchParams.get('ruleId');
-    if (r) setMatcherRuleId(r);
+    if (r) {
+      setMatcherRuleId(r);
+      setCreateOpen(true);
+    }
   }, [searchParams]);
 
+  const resetForm = () => {
+    setReason('');
+    setMatcherRuleId('');
+    setMatcherAppSlug('');
+    setHours(1);
+  };
+
+  const closeModal = () => {
+    setCreateOpen(false);
+    resetForm();
+  };
+
   if (isLoading) return <PageLoader />;
   if (error) return <div className={css.page}>Failed to load silences: {describeApiError(error)}</div>;
 
@@ -58,10 +73,7 @@ export default function SilencesPage() {
         startsAt: now.toISOString(),
         endsAt: endsAt.toISOString(),
       });
-      setReason('');
-      setMatcherRuleId('');
-      setMatcherAppSlug('');
-      setHours(1);
+      closeModal();
       toast({ title: 'Silence created', variant: 'success' });
     } catch (e) {
       toast({ title: 'Create failed', description: describeApiError(e), variant: 'error' });
@@ -109,17 +121,38 @@ export default function SilencesPage() {
               : `${rows.length} active silence${rows.length === 1 ? '' : 's'}`}
           </span>
         </div>
+        <div className={css.pageActions}>
+          <Button variant="primary" onClick={() => setCreateOpen(true)}>New silence</Button>
+        </div>
       </header>
 
-      <section className={sectionStyles.section}>
-        <div
-          style={{
-            display: 'grid',
-            gridTemplateColumns: 'repeat(4, minmax(0, 1fr)) auto',
-            gap: 'var(--space-sm)',
-            alignItems: 'end',
-          }}
-        >
+      {rows.length === 0 ? (
+        <EmptyState
+          icon={<BellOff size={32} />}
+          title="No silences"
+          description="Nothing is currently silenced in this environment."
+          action={
+            <Button variant="primary" onClick={() => setCreateOpen(true)}>Create silence</Button>
+          }
+        />
+      ) : (
+        <div className={`${tableStyles.tableSection} ${css.tableWrap}`}>
+          <DataTable<AlertSilenceResponse & { id: string }>
+            columns={columns}
+            data={rows.map((s) => ({ ...s, id: s.id ?? '' }))}
+            flush
+            fillHeight
+          />
+        </div>
+      )}
+
+      <Modal
+        open={createOpen}
+        onClose={closeModal}
+        title="New silence"
+        size="md"
+      >
+        <div style={{ display: 'grid', gap: 'var(--space-md)' }}>
           <FormField label="Rule ID" hint="Exact rule id (optional)">
             <Input value={matcherRuleId} onChange={(e) => setMatcherRuleId(e.target.value)} />
           </FormField>
@@ -141,28 +174,14 @@ export default function SilencesPage() {
               placeholder="Maintenance window"
             />
           </FormField>
-          <Button variant="primary" size="sm" onClick={onCreate} disabled={create.isPending}>
-            Create silence
-          </Button>
+          <div style={{ display: 'flex', gap: 'var(--space-sm)', justifyContent: 'flex-end' }}>
+            <Button variant="ghost" onClick={closeModal} disabled={create.isPending}>Cancel</Button>
+            <Button variant="primary" onClick={onCreate} disabled={create.isPending}>
+              Create silence
+            </Button>
+          </div>
         </div>
-      </section>
-
-      {rows.length === 0 ? (
-        <EmptyState
-          icon={<BellOff size={32} />}
-          title="No silences"
-          description="Nothing is currently silenced in this environment."
-        />
-      ) : (
-        <div className={`${tableStyles.tableSection} ${css.tableWrap}`}>
-          <DataTable<AlertSilenceResponse & { id: string }>
-            columns={columns}
-            data={rows.map((s) => ({ ...s, id: s.id ?? '' }))}
-            flush
-            fillHeight
-          />
-        </div>
-      )}
+      </Modal>
 
       <ConfirmDialog
         open={!!pendingEnd}