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/components/header-wrapper.tsx

@@ -1,14 +1,16 @@
 'use client'
 
+import { Suspense } from 'react'
 import { Header } from './header'
 import { useSearchParams, useRouter } from 'next/navigation'
 import { useLabels } from '@/context/LabelContext'
 
 interface HeaderWrapperProps {
   onColorFilterChange?: (color: string | null) => void
+  user?: any
 }
 
-export function HeaderWrapper({ onColorFilterChange }: HeaderWrapperProps) {
+function HeaderContent({ onColorFilterChange, user }: HeaderWrapperProps) {
   const searchParams = useSearchParams()
   const router = useRouter()
   const { labels } = useLabels()
@@ -47,6 +49,15 @@ export function HeaderWrapper({ onColorFilterChange }: HeaderWrapperProps) {
       selectedColor={selectedColor}
       onLabelFilterChange={handleLabelFilterChange}
       onColorFilterChange={handleColorFilterChange}
+      user={user}
     />
   )
 }
+
+export function HeaderWrapper(props: HeaderWrapperProps) {
+  return (
+    <Suspense fallback={<div className="h-16 border-b bg-background/95" />}>
+      <HeaderContent {...props} />
+    </Suspense>
+  )
+}