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.

Inizia riproducendo il problema nell’app Copilot su macOS con un pannello laterale di canvas focalizzato, quindi esamina la gestione nativa del menu Edit descritta nel report. Il lavoro è completato quando Cmd+A seleziona i contenuti del pannello focalizzato e Cmd+A seguito da Cmd+C li copia, mentre la selezione con il mouse e Cmd+C continuano a funzionare.

Scritto dal modello di indicizzazione a partire dal testo della issue.