Use specific languages for shiki (#1938)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Refactors code highlighting to use react-shiki/core with a singleton
highlighter and preloaded langs/themes, adds a fallback renderer, bumps
react-shiki, and adds tests for message summarization.
> 
> - **Frontend**
> - **CodeHighlight** (`src/components/chat/CodeHighlight.tsx`): Replace
`useShikiHighlighter` with `ShikiHighlighter` from `react-shiki/core`
using a singleton `createHighlighterCore` and JS regex engine; preload
common languages and GitHub light/dark themes; add `<pre><code>`
fallback while loading.
> - **Tests**
> - Add/expand Vitest suite for `formatMessagesForSummary`
(`src/__tests__/formatMessagesForSummary.test.ts`): covers truncation,
ordering, special chars, undefined content, and edge cases.
> - **Dependencies**
>   - Upgrade `react-shiki` to `^0.9.0` in `package.json`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
b32d224cd21d3c76e77799f2995905e523406bf9. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->





<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Preloaded a specific set of Shiki languages and themes using react-shiki
core, and updated CodeHighlight to use a singleton highlighter. This
reduces bundle size and stabilizes code rendering with a simple fallback
while loading.

- **Refactors**
- Switched to react-shiki/core with ShikiHighlighter and a singleton
highlighter.
- Preloaded common languages
(js/ts/jsx/tsx/html/css/json/markdown/python/etc.) and GitHub light/dark
themes.
- Used the JavaScript regex engine and added a plain <pre><code>
fallback until the highlighter is ready.

- **Dependencies**
  - Upgraded react-shiki to ^0.9.0.

<sup>Written for commit b32d224cd21d3c76e77799f2995905e523406bf9.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

package.json

@@ -158,7 +158,7 @@
     "react-dom": "^19.2.1",
     "react-markdown": "^10.1.0",
     "react-resizable-panels": "^2.1.7",
-    "react-shiki": "^0.5.2",
+    "react-shiki": "^0.9.0",
     "recast": "^0.23.11",
     "remark-gfm": "^4.0.1",
     "shell-env": "^4.0.1",