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/avatar.tsx

@@ -0,0 +1,50 @@
+"use client"
+
+import * as React from "react"
+import * as AvatarPrimitive from "@radix-ui/react-avatar"
+
+import { cn } from "@/lib/utils"
+
+const Avatar = React.forwardRef<
+  React.ElementRef<typeof AvatarPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof AvatarPrimitive.Root>
+>(({ className, ...props }, ref) => (
+  <AvatarPrimitive.Root
+    ref={ref}
+    className={cn(
+      "relative flex h-10 w-10 shrink-0 overflow-hidden rounded-full",
+      className
+    )}
+    {...props}
+  />
+))
+Avatar.displayName = AvatarPrimitive.Root.displayName
+
+const AvatarImage = React.forwardRef<
+  React.ElementRef<typeof AvatarPrimitive.Image>,
+  React.ComponentPropsWithoutRef<typeof AvatarPrimitive.Image>
+>(({ className, ...props }, ref) => (
+  <AvatarPrimitive.Image
+    ref={ref}
+    className={cn("aspect-square h-full w-full", className)}
+    {...props}
+  />
+))
+AvatarImage.displayName = AvatarPrimitive.Image.displayName
+
+const AvatarFallback = React.forwardRef<
+  React.ElementRef<typeof AvatarPrimitive.Fallback>,
+  React.ComponentPropsWithoutRef<typeof AvatarPrimitive.Fallback>
+>(({ className, ...props }, ref) => (
+  <AvatarPrimitive.Fallback
+    ref={ref}
+    className={cn(
+      "flex h-full w-full items-center justify-center rounded-full bg-muted",
+      className
+    )}
+    {...props}
+  />
+))
+AvatarFallback.displayName = AvatarPrimitive.Fallback.displayName
+
+export { Avatar, AvatarImage, AvatarFallback }