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/label-selector.tsx

@@ -94,7 +94,7 @@ export function LabelSelector({
                       e.preventDefault()
                       handleToggleLabel(label.name)
                     }}
-                    className="flex items-center gap-2 p-2 rounded-sm hover:bg-gray-100 dark:hover:bg-zinc-800 cursor-pointer text-sm"
+                    className="flex items-center gap-2 p-2 rounded-sm cursor-pointer text-sm hover:bg-accent hover:text-accent-foreground"
                   >
                     <div className={cn(
                       "h-4 w-4 border rounded flex items-center justify-center transition-colors",
@@ -113,7 +113,7 @@ export function LabelSelector({
                     e.preventDefault()
                     handleCreateLabel()
                   }}
-                  className="flex items-center gap-2 p-2 rounded-sm hover:bg-gray-100 dark:hover:bg-zinc-800 cursor-pointer text-sm border-t mt-1"
+                  className="flex items-center gap-2 p-2 rounded-sm cursor-pointer text-sm border-t mt-1 font-medium hover:bg-accent hover:text-accent-foreground"
                 >
                   <Plus className="h-4 w-4" />
                   <span>Create "{search}"</span>