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/globals.css

@@ -1,8 +1,21 @@
 @import "tailwindcss";
+@plugin "@tailwindcss/typography";
 @import "tw-animate-css";
 
 @custom-variant dark (&:is(.dark *));
 
+/* Custom Prose overrides for compact notes */
+@utility prose-compact {
+  & :where(h1, h2, h3, h4) {
+    margin-top: 0.5rem;
+    margin-bottom: 0.25rem;
+  }
+  & :where(p, ul, ol, li) {
+    margin-top: 0.25rem;
+    margin-bottom: 0.25rem;
+  }
+}
+
 @theme inline {
   --color-background: var(--background);
   --color-foreground: var(--foreground);