feat(classes): admin manage pages + sidebar

/classes/manage (guarded by classes.manage) lists classes with inline
create/edit, archive/restore, delete (refused once sessions exist), an
optional grants-certification picker and a generic payment link, plus
per-class session scheduling with cancel/delete. Sidebar gets a "Manage
classes" link in the Admin block. The member-facing catalog/signup is
the next phase.

app/(app)/classes/manage/page.tsx

@@ -0,0 +1,36 @@
+import { requireClassesManagerPage } from '@/lib/classes-guard'
+import { ClassesManageClient } from './classes-manage-client'
+
+export const dynamic = 'force-dynamic'
+
+export default async function ClassesManagePage() {
+  const { supabase, member } = await requireClassesManagerPage()
+
+  const [{ data: classes }, { data: sessions }, { data: certs }] = await Promise.all([
+    supabase
+      .from('classes')
+      .select('id, title, description, payment_link, capacity, is_active, grants_certification_id, created_at, updated_at')
+      .eq('space_id', member.space_id)
+      .order('is_active', { ascending: false })
+      .order('title', { ascending: true }),
+    supabase
+      .from('class_sessions')
+      .select('id, class_id, starts_at, ends_at, location, capacity, status, notes')
+      .eq('space_id', member.space_id)
+      .order('starts_at', { ascending: true }),
+    supabase
+      .from('certifications')
+      .select('id, name, is_active')
+      .eq('space_id', member.space_id)
+      .eq('is_active', true)
+      .order('name', { ascending: true }),
+  ])
+
+  return (
+    <ClassesManageClient
+      initialClasses={classes ?? []}
+      initialSessions={sessions ?? []}
+      certs={certs ?? []}
+    />
+  )
+}