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/app/api/v1/client-groups/[id]/members/handlers.ts

@@ -0,0 +1,31 @@
+import { NextResponse } from 'next/server';
+
+import { type RouteHandler } from '@/lib/api/helpers';
+import { parseBody } from '@/lib/api/route-helpers';
+import { errorResponse } from '@/lib/errors';
+import { listGroupMembers, setGroupMembers } from '@/lib/services/client-groups.service';
+import { setGroupMembersSchema } from '@/lib/validators/client-groups';
+
+export const getMembersHandler: RouteHandler = async (req, ctx, params) => {
+  try {
+    const members = await listGroupMembers(params.id!, ctx.portId);
+    return NextResponse.json({ data: members, total: members.length });
+  } catch (error) {
+    return errorResponse(error);
+  }
+};
+
+export const putMembersHandler: RouteHandler = async (req, ctx, params) => {
+  try {
+    const { clientIds } = await parseBody(req, setGroupMembersSchema);
+    await setGroupMembers(params.id!, ctx.portId, clientIds, {
+      userId: ctx.userId,
+      portId: ctx.portId,
+      ipAddress: ctx.ipAddress,
+      userAgent: ctx.userAgent,
+    });
+    return new NextResponse(null, { status: 204 });
+  } catch (error) {
+    return errorResponse(error);
+  }
+};