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/markdown-content.tsx

@@ -2,6 +2,9 @@
 
 import ReactMarkdown from 'react-markdown'
 import remarkGfm from 'remark-gfm'
+import remarkMath from 'remark-math'
+import rehypeKatex from 'rehype-katex'
+import 'katex/dist/katex.min.css'
 
 interface MarkdownContentProps {
   content: string
@@ -10,8 +13,16 @@ interface MarkdownContentProps {
 
 export function MarkdownContent({ content, className = '' }: MarkdownContentProps) {
   return (
-    <div className={`prose prose-sm dark:prose-invert max-w-none ${className}`}>
-      <ReactMarkdown remarkPlugins={[remarkGfm]}>
+    <div className={`prose prose-sm prose-compact dark:prose-invert max-w-none break-words ${className}`}>
+      <ReactMarkdown
+        remarkPlugins={[remarkGfm, remarkMath]}
+        rehypePlugins={[rehypeKatex]}
+        components={{
+          a: ({ node, ...props }) => (
+            <a {...props} className="text-blue-500 hover:underline" target="_blank" rel="noopener noreferrer" />
+          )
+        }}
+      >
         {content}
       </ReactMarkdown>
     </div>