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/(main)/settings/loading.tsx

@@ -1,26 +0,0 @@
-export default function SettingsLoading() {
-  return (
-    <div className="space-y-6 animate-pulse">
-      <div>
-        <div className="h-9 w-64 bg-muted rounded-md mb-2" />
-        <div className="h-4 w-96 bg-muted rounded-md" />
-      </div>
-      {[1, 2, 3].map((i) => (
-        <div key={i} className="rounded-lg border border-border p-6 space-y-4">
-          <div className="flex items-center gap-3">
-            <div className="h-8 w-8 bg-muted rounded-full" />
-            <div className="h-5 w-40 bg-muted rounded-md" />
-          </div>
-          <div className="h-px bg-border" />
-          <div className="flex items-center justify-between p-4 rounded-lg bg-muted/30">
-            <div className="space-y-2">
-              <div className="h-4 w-32 bg-muted rounded" />
-              <div className="h-3 w-56 bg-muted rounded" />
-            </div>
-            <div className="h-6 w-11 bg-muted rounded-full" />
-          </div>
-        </div>
-      ))}
-    </div>
-  )
-}