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/(main)/layout.tsx

@@ -0,0 +1,23 @@
+import { HeaderWrapper } from "@/components/header-wrapper";
+import { Sidebar } from "@/components/sidebar";
+import { auth } from "@/auth";
+
+export default async function MainLayout({
+  children,
+}: Readonly<{
+  children: React.ReactNode;
+}>) {
+  const session = await auth();
+
+  return (
+    <div className="flex min-h-screen flex-col">
+      <HeaderWrapper user={session?.user} />
+      <div className="flex flex-1">
+        <Sidebar className="shrink-0 border-r" user={session?.user} />
+        <main className="flex-1">
+          {children}
+        </main>
+      </div>
+    </div>
+  );
+}