[BUG] Desktop Code tab: bold (**…**) is invisible for Chinese text in Markdown preview and chat (Windows)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

In the Claude desktop app (Code tab) on Windows, Chinese text inside `**…**` is rendered at regular weight, indistinguishable from body text. This happens both in the Markdown file preview and in chat replies. `__…__` behaves the same; `***…***` shows italic only; English bold is only slightly heavier. Headings (`###` / `####`) are the only way Chinese text appears bold. The same file renders bold correctly in VS Code preview and Feishu.

### What Should Happen?

Chinese text inside `**…**` should be rendered bold, like English text and like headings.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Create a Markdown file with (full test file in Additional Information):
```markdown
**研究**
这是普通文字，**这是重点短语**，后面继续。
plain, **bold English**, plain.
#### 研究
```
2. Open it in the Code tab file preview (or ask Claude to reply with the same text in chat).
3. Compare the weight of the Chinese bold text with body text and with the heading.

### Claude Model

Opus

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

Claude desktop for Windows 2.2553.1.0 (Code tab); Claude Code CLI 2.1.276

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Other

### Additional Information

Likely cause: emphasis weight seems to be applied in a way the CJK fallback font (Microsoft YaHei) cannot honor, e.g. through the Latin variable font's wght axis via font-variation-settings, so CJK glyphs stay regular. Setting a real font-weight (600 or more) on strong/b, or allowing font-synthesis for weight, should fix it.
Impact: Chinese-language users lose all inline emphasis in chat replies and file previews.

Full test file (letters match the screenshot; only F and G, which are headings, render bold; D is only slightly heavier; H shows italic only):

```markdown
A. Standalone Chinese bold line:

**研究**

B. Bold label followed by body text:

**为什么**：后面这句是普通正文。

C. Chinese bold inside a sentence: 这是普通文字，**这是重点短语**，后面继续普通文字。

D. English bold: plain text, **bold English words**, plain again.

E. Underscore bold: __研究__

F. Level-3 heading:

### 研究

G. Level-4 heading:

#### 研究

H. Bold italic: ***研究***

I. Numbered list item:

1. **编号条目里的加粗**

J. Bullet list item:

- **列表条目里的加粗**：后面是普通文字。
```

Reproduce the Markdown in the Code tab file preview and chat on Windows, comparing Chinese and English emphasis. No source files or tests are named, so trace the Markdown strong/emphasis rendering and CJK font handling from those entry points. Done means Chinese text in **…**, __…__, and combined emphasis renders visibly bold in both contexts without regressing headings or English text.

Written by the indexing model from the issue text.