feat(documents): detail page with signers, watchers, activity, actions

Replaces the PR4 stub at /documents/[id] with the full Phase A detail
view: gradient header strip, status-aware action bar (Cancel /
Download / Email signatories), per-signer remind + copy-link, watcher
list with remove, and activity timeline. Adds the supporting endpoints
(cancel, compose-completion-email, watchers GET/POST/DELETE) and
listDocumentWatchers / addDocumentWatcher / removeDocumentWatcher
service helpers. The document GET now serves the aggregator shape
when ?detail=true.

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

src/app/(dashboard)/[portSlug]/documents/[id]/page.tsx

@@ -1,6 +1,4 @@
-import Link from 'next/link';
-import { Button } from '@/components/ui/button';
-import { PageHeader } from '@/components/shared/page-header';
+import { DocumentDetail } from '@/components/documents/document-detail';
 
 interface PageProps {
   params: Promise<{ portSlug: string; id: string }>;
@@ -8,17 +6,5 @@ interface PageProps {
 
 export default async function DocumentDetailPage({ params }: PageProps) {
   const { portSlug, id } = await params;
-  return (
-    <div className="flex flex-col gap-4">
-      <PageHeader
-        title="Document detail"
-        description={`Document ${id} — full detail view ships in PR5 of the Phase A rollout.`}
-        actions={
-          <Button asChild variant="outline">
-            <Link href={`/${portSlug}/documents`}>Back to documents</Link>
-          </Button>
-        }
-      />
-    </div>
-  );
+  return <DocumentDetail documentId={id} portSlug={portSlug} />;
 }