Support Ctrl+mouse wheel zoom in markdown canvases

## Feature request

Please support zooming markdown canvas content with **Ctrl + mouse wheel** on Windows.

## Current behavior

The GitHub Copilot app markdown canvases can be zoomed with keyboard shortcuts such as **Ctrl + +** and **Ctrl + -**, but holding **Ctrl** and scrolling the mouse wheel does not zoom the markdown canvas content.

## Expected behavior

When focus is in a markdown canvas, **Ctrl + mouse wheel up/down** should zoom in/out, matching the existing keyboard zoom behavior.

## Why this matters

Markdown canvases are often used for plans, notes, and rendered markdown review. I frequently want to temporarily make the text larger while reading or editing markdown. Ctrl+mouse-wheel zoom is a common Windows interaction pattern in browsers, Office apps, and editor/document surfaces, so it is an expected shortcut for text-heavy views.

## Possible scope

The narrow request is for markdown-related canvases, including plan markdown and markdown viewer/editor canvases. It may also be worth considering whether the same interaction should apply consistently across all text-heavy Copilot app canvases.

没有指定文件或测试。首先定位 markdown canvas 的实现以及现有的 Ctrl + + 和 Ctrl + - 缩放处理，然后跟踪鼠标滚轮输入如何到达 canvas。完成的标准是，Ctrl + 鼠标滚轮向上/向下能够以与现有键盘行为一致的方式更改 markdown canvas 的缩放级别，并明确其他文本密集型 canvas 的适用范围。

由索引模型根据 Issue 内容生成。