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.

No files or tests are named. Start by locating the markdown canvas implementation and the existing Ctrl + + and Ctrl + - zoom handling, then trace how mouse-wheel input reaches the canvas. Done means Ctrl + mouse wheel up/down changes markdown canvas zoom consistently with the existing keyboard behavior, with the scope for other text-heavy canvases clarified.

索引モデルが issue の本文から書いたものです。