fix(ui): render markdown horizontal rules (#498)

## Summary
- render Markdown horizontal rules instead of hiding them in message
content
- keep the existing Markdown parser behavior and style the generated hr
with the shared border token

Solves #491

## Validation
- npm run typecheck --workspace @codenomad/ui
- npm run build:linux --workspace @neuralnomads/codenomad-electron-app

## Test release artifact
Built locally for Linux; artifacts are ignored and not included in this
PR:
- packages/electron-app/release/CodeNomad-Electron-linux-x64-0.16.0.zip
-
packages/electron-app/release/CodeNomad-Electron-linux-x86_64-0.16.0.AppImage

Co-authored-by: Shantur Rathore <i@shantur.com>
Co-authored-by: Pascal André <pascalandr@gmail.com>

packages/ui/src/styles/markdown.css

@@ -103,7 +103,10 @@
   }
 
   .markdown-body hr {
-    display: none;
+    height: 1px;
+    margin: 0.9rem 0;
+    background-color: var(--border-base);
+    border: 0;
   }
 
   .markdown-body strong {