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/implementation-artifacts/sprint-status.yaml

@@ -37,9 +37,9 @@ development_status:
   2-3-validation-des-suggestions-par-l-utilisateur: backlog
   epic-2-retrospective: optional
 
-  epic-3: backlog
-  3-1-indexation-vectorielle-automatique: backlog
-  3-2-recherche-semantique-par-intention: backlog
+  epic-3: in-progress
+  3-1-indexation-vectorielle-automatique: done
+  3-2-recherche-semantique-par-intention: in-progress
   3-3-vue-de-recherche-hybride: backlog
   epic-3-retrospective: optional