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/(auth)/login/page.tsx

@@ -0,0 +1,17 @@
+import { LoginForm } from '@/components/login-form';
+import { getSystemConfig } from '@/lib/config';
+ 
+export default async function LoginPage() {
+  const config = await getSystemConfig();
+  
+  // Default to true unless explicitly disabled in DB or Env
+  const allowRegister = config.ALLOW_REGISTRATION !== 'false' && process.env.ALLOW_REGISTRATION !== 'false';
+
+  return (
+    <main className="flex items-center justify-center md:h-screen">
+      <div className="relative mx-auto flex w-full max-w-[400px] flex-col space-y-2.5 p-4 md:-mt-32">
+        <LoginForm allowRegister={allowRegister} />
+      </div>
+    </main>
+  );
+}