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/ui/label.tsx

@@ -0,0 +1,16 @@
+import * as React from "react"
+import { cn } from "@/lib/utils"
+
+export interface LabelProps extends React.LabelHTMLAttributes<HTMLLabelElement> {}
+
+export function Label({ className, ...props }: LabelProps) {
+  return (
+    <label
+      className={cn(
+        "text-sm font-medium leading-none peer-disabled:cursor-not-allowed peer-disabled:opacity-70",
+        className
+      )}
+      {...props}
+    />
+  )
+}