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/context/LabelContext.tsx

@@ -32,7 +32,10 @@ export function LabelProvider({ children }: { children: ReactNode }) {
   const fetchLabels = async () => {
     try {
       setLoading(true)
-      const response = await fetch('/api/labels', { cache: 'no-store' })
+      const response = await fetch('/api/labels', {
+        cache: 'no-store',
+        credentials: 'include'
+      })
       const data = await response.json()
       if (data.success && data.data) {
         setLabels(data.data)