fix: react-doctor no-barrel-import 20→1 (direct design-system imports)

Convert ./design-system barrel imports to direct file paths across
19 admin components. Remaining 1 (db/client.ts) is Drizzle schema
which legitimately requires namespace import for drizzle() helper.

src/components/admin/HeadgateEditForm.tsx

@@ -3,8 +3,8 @@
 import { useState } from "react";
 import { useRouter } from "next/navigation";
 import { updateWaterHeadgate, deleteWaterHeadgate } from "@/actions/water-log/admin";
-import { AdminInput, AdminTextInput, AdminSelect } from "./design-system";
-import { AdminButton } from "./design-system";
+import { AdminInput,  AdminTextInput, AdminSelect  } from "./design-system/AdminFormElements";
+import AdminButton from "./design-system/AdminButton";
 
 type Headgate = {
   id: string;