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/profile.ts

@@ -0,0 +1,100 @@
+'use server'
+
+import { revalidatePath } from 'next/cache'
+import prisma from '@/lib/prisma'
+import { auth } from '@/auth'
+import bcrypt from 'bcryptjs'
+import { z } from 'zod'
+
+const ProfileSchema = z.object({
+  name: z.string().min(2, "Name must be at least 2 characters"),
+  email: z.string().email().optional(), // Email change might require verification logic, keeping it simple for now or read-only
+})
+
+const PasswordSchema = z.object({
+  currentPassword: z.string().min(1, "Current password is required"),
+  newPassword: z.string().min(6, "New password must be at least 6 characters"),
+  confirmPassword: z.string().min(6, "Confirm password must be at least 6 characters"),
+}).refine((data) => data.newPassword === data.confirmPassword, {
+  message: "Passwords don't match",
+  path: ["confirmPassword"],
+})
+
+export async function updateProfile(data: { name: string }) {
+  const session = await auth()
+  if (!session?.user?.id) throw new Error('Unauthorized')
+
+  const validated = ProfileSchema.safeParse(data)
+  if (!validated.success) {
+    return { error: validated.error.flatten().fieldErrors }
+  }
+
+  try {
+    await prisma.user.update({
+      where: { id: session.user.id },
+      data: { name: validated.data.name },
+    })
+    revalidatePath('/settings/profile')
+    return { success: true }
+  } catch (error) {
+    return { error: { _form: ['Failed to update profile'] } }
+  }
+}
+
+export async function changePassword(formData: FormData) {
+  const session = await auth()
+  if (!session?.user?.id) throw new Error('Unauthorized')
+
+  const rawData = {
+    currentPassword: formData.get('currentPassword'),
+    newPassword: formData.get('newPassword'),
+    confirmPassword: formData.get('confirmPassword'),
+  }
+
+  const validated = PasswordSchema.safeParse(rawData)
+  if (!validated.success) {
+    return { error: validated.error.flatten().fieldErrors }
+  }
+
+  const { currentPassword, newPassword } = validated.data
+
+  const user = await prisma.user.findUnique({
+    where: { id: session.user.id },
+  })
+
+  if (!user || !user.password) {
+    return { error: { _form: ['User not found'] } }
+  }
+
+  const passwordsMatch = await bcrypt.compare(currentPassword, user.password)
+  if (!passwordsMatch) {
+    return { error: { currentPassword: ['Incorrect current password'] } }
+  }
+
+  const hashedPassword = await bcrypt.hash(newPassword, 10)
+
+  try {
+    await prisma.user.update({
+      where: { id: session.user.id },
+      data: { password: hashedPassword },
+    })
+    return { success: true }
+  } catch (error) {
+    return { error: { _form: ['Failed to change password'] } }
+  }
+}
+
+export async function updateTheme(theme: string) {
+  const session = await auth()
+  if (!session?.user?.id) return { error: 'Unauthorized' }
+
+  try {
+    await prisma.user.update({
+      where: { id: session.user.id },
+      data: { theme },
+    })
+    return { success: true }
+  } catch (error) {
+    return { error: 'Failed to update theme' }
+  }
+}