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

_bmad-output/planning-artifacts/implementation-readiness-report-2026-01-09.md

@@ -0,0 +1,27 @@
+---
+stepsCompleted: [1]
+---
+
+# Implementation Readiness Assessment Report
+
+**Date:** 2026-01-09
+**Project:** Keep
+
+## 1. Document Inventory
+
+### PRD Documents
+- prd.md
+- prd-executive-summary.md
+- prd-web-app-requirements.md
+- prd-auth-admin.md
+
+### Architecture Documents
+- ⚠️ MISSING
+
+### Epics & Stories Documents
+- epics.md
+
+### UX Design Documents
+- ⚠️ MISSING
+
+---