fix: improve note interactions and markdown LaTeX support

## Bug Fixes

### Note Card Actions
- Fix broken size change functionality (missing state declaration)
- Implement React 19 useOptimistic for instant UI feedback
- Add startTransition for non-blocking updates
- Ensure smooth animations without page refresh
- All note actions now work: pin, archive, color, size, checklist

### Markdown LaTeX Rendering
- Add remark-math and rehype-katex plugins
- Support inline equations with dollar sign syntax
- Support block equations with double dollar sign syntax
- Import KaTeX CSS for proper styling
- Equations now render correctly instead of showing raw LaTeX

## Technical Details

- Replace undefined currentNote references with optimistic state
- Add optimistic updates before server actions for instant feedback
- Use router.refresh() in transitions for smart cache invalidation
- Install remark-math, rehype-katex, and katex packages

## Testing

- Build passes successfully with no TypeScript errors
- Dev server hot-reloads changes correctly

keep-notes/app/(main)/admin/settings/page.tsx

@@ -0,0 +1,21 @@
+import { auth } from '@/auth'
+import { redirect } from 'next/navigation'
+import { getSystemConfig } from '@/app/actions/admin-settings'
+import { AdminSettingsForm } from './admin-settings-form'
+
+export default async function AdminSettingsPage() {
+  const session = await auth()
+  
+  if ((session?.user as any)?.role !== 'ADMIN') {
+    redirect('/')
+  }
+
+  const config = await getSystemConfig()
+
+  return (
+    <div className="container max-w-4xl mx-auto py-10 px-4">
+      <h1 className="text-3xl font-bold mb-8">System Configuration</h1>
+      <AdminSettingsForm config={config} />
+    </div>
+  )
+}