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.ts

@@ -0,0 +1,120 @@
+'use server'
+
+import { revalidatePath } from 'next/cache'
+import prisma from '@/lib/prisma'
+import { auth } from '@/auth'
+import bcrypt from 'bcryptjs'
+import { z } from 'zod'
+
+// Schema pour la création d'utilisateur
+const CreateUserSchema = z.object({
+  name: z.string().min(2, "Name must be at least 2 characters"),
+  email: z.string().email("Invalid email address"),
+  password: z.string().min(6, "Password must be at least 6 characters"),
+  role: z.enum(["USER", "ADMIN"]).default("USER"),
+})
+
+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 getUsers() {
+  await checkAdmin()
+  try {
+    const users = await prisma.user.findMany({
+      orderBy: { createdAt: 'desc' },
+      select: {
+        id: true,
+        name: true,
+        email: true,
+        role: true,
+        createdAt: true,
+      }
+    })
+    return users
+  } catch (error) {
+    console.error('Failed to fetch users:', error)
+    throw new Error('Failed to fetch users')
+  }
+}
+
+export async function createUser(formData: FormData) {
+  await checkAdmin()
+
+  const rawData = {
+    name: formData.get('name'),
+    email: formData.get('email'),
+    password: formData.get('password'),
+    role: formData.get('role'),
+  }
+
+  const validatedFields = CreateUserSchema.safeParse(rawData)
+
+  if (!validatedFields.success) {
+    return {
+      error: validatedFields.error.flatten().fieldErrors,
+    }
+  }
+
+  const { name, email, password, role } = validatedFields.data
+  const hashedPassword = await bcrypt.hash(password, 10)
+
+  try {
+    await prisma.user.create({
+      data: {
+        name,
+        email: email.toLowerCase(),
+        password: hashedPassword,
+        role,
+      },
+    })
+    revalidatePath('/admin')
+    return { success: true }
+  } catch (error: any) {
+    if (error.code === 'P2002') {
+      return { error: { email: ['Email already exists'] } }
+    }
+    return { error: { _form: ['Failed to create user'] } }
+  }
+}
+
+export async function deleteUser(userId: string) {
+  const session = await checkAdmin()
+  
+  if (session.user?.id === userId) {
+    throw new Error("You cannot delete your own account")
+  }
+
+  try {
+    await prisma.user.delete({
+      where: { id: userId },
+    })
+    revalidatePath('/admin')
+    return { success: true }
+  } catch (error) {
+    throw new Error('Failed to delete user')
+  }
+}
+
+export async function updateUserRole(userId: string, newRole: string) {
+  const session = await checkAdmin()
+
+  if (session.user?.id === userId) {
+    throw new Error("You cannot change your own role")
+  }
+
+  try {
+    await prisma.user.update({
+      where: { id: userId },
+      data: { role: newRole },
+    })
+    revalidatePath('/admin')
+    return { success: true }
+  } catch (error) {
+    throw new Error('Failed to update role')
+  }
+}