fix: resolve React Error #310 and refactor admin section

- Fix React bug #33580: remove Suspense boundaries co-located with Link components
- Delete settings/loading.tsx and admin/loading.tsx (root cause of race condition)
- Convert all admin navigation from Next.js Link to anchor tags
- Move admin pages to dedicated (admin) route group
- Add AdminHeader matching main header visual design
- Add AdminSidebar with anchor-based navigation
- Add /api/admin/models route handler (replaces server actions for GET)
- Add /api/debug/client-error for server-side browser error reporting
- Add useNoteRefreshOptional() to fix crash in AdminHeader
- Hide Admin Dashboard menu for non-admin users
- Change app icons from yellow to blue (#3A7CA5) matching brand primary
- Fix admin search bar width to match main header

Made-with: Cursor

memento-note/app/(admin)/admin/layout.tsx

@@ -0,0 +1,22 @@
+import { AdminHeader } from '@/components/admin-header'
+import { AdminSidebar } from '@/components/admin-sidebar'
+import { AdminContentArea } from '@/components/admin-content-area'
+
+// Auth is enforced solely by middleware (auth.config.ts → authorized callback).
+// All cross-group navigation (admin ↔ main) uses <a> tags (full page reload)
+// to avoid React Error #310 caused by Next.js 16.x route-group transition bug.
+export default function AdminLayout({
+  children,
+}: {
+  children: React.ReactNode
+}) {
+  return (
+    <div className="bg-background-light dark:bg-background-dark font-display text-slate-900 dark:text-white overflow-hidden h-screen flex flex-col">
+      <AdminHeader />
+      <div className="flex flex-1 overflow-hidden">
+        <AdminSidebar />
+        <AdminContentArea>{children}</AdminContentArea>
+      </div>
+    </div>
+  )
+}