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/register.ts

@@ -4,6 +4,7 @@ import bcrypt from 'bcryptjs';
 import prisma from '@/lib/prisma';
 import { z } from 'zod';
 import { redirect } from 'next/navigation';
+import { getSystemConfig } from '@/lib/config';
 
 const RegisterSchema = z.object({
   email: z.string().email(),
@@ -12,6 +13,14 @@ const RegisterSchema = z.object({
 });
 
 export async function register(prevState: string | undefined, formData: FormData) {
+  // Check if registration is allowed
+  const config = await getSystemConfig();
+  const allowRegister = config.ALLOW_REGISTRATION !== 'false' && process.env.ALLOW_REGISTRATION !== 'false';
+
+  if (!allowRegister) {
+    return 'Registration is currently disabled by the administrator.';
+  }
+
   const validatedFields = RegisterSchema.safeParse({
     email: formData.get('email'),
     password: formData.get('password'),