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/context/NoteRefreshContext.tsx

@@ -0,0 +1,33 @@
+'use client'
+
+import { createContext, useContext, useState, useCallback } from 'react'
+
+interface NoteRefreshContextType {
+  refreshKey: number
+  triggerRefresh: () => void
+}
+
+const NoteRefreshContext = createContext<NoteRefreshContextType | undefined>(undefined)
+
+export function NoteRefreshProvider({ children }: { children: React.ReactNode }) {
+  const [refreshKey, setRefreshKey] = useState(0)
+
+  const triggerRefresh = useCallback(() => {
+    console.log('[NOTE_REFRESH] Triggering refresh, key:', refreshKey, '->', refreshKey + 1)
+    setRefreshKey(prev => prev + 1)
+  }, [refreshKey])
+
+  return (
+    <NoteRefreshContext.Provider value={{ refreshKey, triggerRefresh }}>
+      {children}
+    </NoteRefreshContext.Provider>
+  )
+}
+
+export function useNoteRefresh() {
+  const context = useContext(NoteRefreshContext)
+  if (!context) {
+    throw new Error('useNoteRefresh must be used within NoteRefreshProvider')
+  }
+  return context
+}