Cmd+A (Select All) does not work in canvas side panels on macOS

### Short summary

Cmd+A (Select All) does nothing when focus is in a canvas side panel on macOS.

### Affected version or release

Copilot app, macOS (Darwin). Version not captured at the time of the report.

### Installation context

Local desktop app, canvas side panel open next to a project session.

### What happened?

While viewing content in a canvas side panel, I pressed **Cmd+A** to select the page contents so I could copy them. Nothing was selected.

Manually drag-selecting the text with the mouse and then pressing **Cmd+C** works fine, so copying is possible — but for anything longer than a screen it's tedious and error-prone.

This looks like the app's native macOS **Edit** menu has no **Select All** item bound to Cmd+A, so the shortcut is never routed into the webview hosting the canvas. Drag-select and Cmd+C are native webview behaviors that work without a menu binding, which is consistent with only Cmd+A being broken.

### Steps to reproduce

1. On macOS, open the Copilot app.
2. Open any canvas in the side panel that renders a page of text/content.
3. Click into the canvas panel to give it focus.
4. Press **Cmd+A**.
5. Observe that nothing is selected.
6. Drag-select some text with the mouse and press **Cmd+C** — this works, confirming copy itself is functional.

### Expected behavior

Cmd+A should select the contents of the focused canvas panel, matching standard macOS behavior and the rest of the app. Cmd+A followed by Cmd+C should copy the panel's contents.

### Additional context

- Platform: macOS (Darwin)
- Only Cmd+A appears affected; mouse selection and Cmd+C behave normally.
- Likely fix is registering a **Select All** accelerator in the app's native Edit menu so the shortcut reaches the webview.

首先，在 macOS 上的 Copilot 应用中使用一个处于焦点状态的 canvas 侧边面板重现该问题，然后检查报告中描述的原生 Edit 菜单处理。完成的标准是，Cmd+A 能选中处于焦点状态的面板内容，随后按 Cmd+C 能复制这些内容，同时鼠标选择和 Cmd+C 仍能正常工作。

由索引模型根据 Issue 内容生成。