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.

Es werden keine Dateien oder Tests genannt. Beginne damit, die Implementierung des markdown canvas und die vorhandene Zoom-Verarbeitung für Ctrl + + und Ctrl + - zu finden, und verfolge anschließend, wie die Mausradeingabe den Canvas erreicht. Als erledigt gilt die Aufgabe, wenn Ctrl + Mausrad nach oben/unten den Zoom des markdown canvas konsistent mit dem bestehenden Tastaturverhalten ändert und der Umfang für andere textlastige Canvases geklärt ist.

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