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/actions/admin-settings.ts

@@ -0,0 +1,59 @@
+'use server'
+
+import { revalidatePath } from 'next/cache'
+import prisma from '@/lib/prisma'
+import { auth } from '@/auth'
+import { sendEmail } from '@/lib/mail'
+
+async function checkAdmin() {
+  const session = await auth()
+  if (!session?.user?.id || (session.user as any).role !== 'ADMIN') {
+    throw new Error('Unauthorized: Admin access required')
+  }
+  return session
+}
+
+export async function testSMTP() {
+  const session = await checkAdmin()
+  const email = session.user?.email
+  
+  if (!email) throw new Error("No admin email found")
+
+  const result = await sendEmail({
+    to: email,
+    subject: "Memento SMTP Test",
+    html: "<p>This is a test email from your Memento instance. <strong>SMTP is working!</strong></p>"
+  })
+
+  return result
+}
+
+export async function getSystemConfig() {
+  await checkAdmin()
+  const configs = await prisma.systemConfig.findMany()
+  return configs.reduce((acc, conf) => {
+    acc[conf.key] = conf.value
+    return acc
+  }, {} as Record<string, string>)
+}
+
+export async function updateSystemConfig(data: Record<string, string>) {
+  await checkAdmin()
+  
+  try {
+    const operations = Object.entries(data).map(([key, value]) => 
+      prisma.systemConfig.upsert({
+        where: { key },
+        update: { value },
+        create: { key, value }
+      })
+    )
+    
+    await prisma.$transaction(operations)
+    revalidatePath('/admin/settings')
+    return { success: true }
+  } catch (error) {
+    console.error('Failed to update settings:', error)
+    return { error: 'Failed to update settings' }
+  }
+}