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/public/icons/icon-192.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 192 192">
+  <rect width="192" height="192" fill="#f59e0b" rx="40"/>
+  <path d="M50 50 h92 v92 h-92 z" fill="#fff"/>
+</svg>

keep-notes/public/icons/icon-512.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" fill="#f59e0b" rx="100"/>
+  <path d="M150 150 h212 v212 h-212 z" fill="#fff"/>
+  <path d="M150 150 h212" stroke="#fff3cd" stroke-width="20"/>
+</svg>

keep-notes/public/manifest.json

@@ -0,0 +1,23 @@
+{
+  "name": "Memento Notes",
+  "short_name": "Memento",
+  "description": "A smart, local-first note taking app with AI capabilities.",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#ffffff",
+  "theme_color": "#f59e0b",
+  "icons": [
+    {
+      "src": "/icons/icon-192.svg",
+      "sizes": "192x192",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/icons/icon-512.svg",
+      "sizes": "512x512",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}