feat(client-groups): CM-1 API routes + UI (list, member viewer, copy-emails)

- /api/v1/client-groups (list/create), /[id] (get/patch/delete),
  /[id]/members (get/set) — route.ts + handlers.ts split, client_groups perms
- Client Groups list page (grid + create dialog) and detail page
  (member viewer, per-row copy email, "Copy all emails" → To:-bar format,
  manage-members picker over /api/v1/clients)
- Sidebar nav entry (UsersRound icon)

tsc clean, lint 0 errors, prod build green. Completes CM-1 (Mailchimp push
still deferred until client creds/account).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/layout/sidebar.tsx

@@ -7,6 +7,7 @@ import { usePathname } from 'next/navigation';
 import {
   LayoutDashboard,
   Users,
+  UsersRound,
   Bookmark,
   Anchor,
   KeyRound,
@@ -113,6 +114,7 @@ function buildNavSections(portSlug: string | undefined): NavSection[] {
       items: [
         { href: `${base}/dashboard`, label: 'Dashboard', icon: LayoutDashboard },
         { href: `${base}/clients`, label: 'Clients', icon: Users },
+        { href: `${base}/client-groups`, label: 'Client Groups', icon: UsersRound },
         { href: `${base}/yachts`, label: 'Yachts', icon: Ship },
         { href: `${base}/companies`, label: 'Companies', icon: Building2 },
         { href: `${base}/interests`, label: 'Interests', icon: Bookmark },