Slide navigation with arrow keys not working on Windows

### Describe the bug

Starting up podium on Windows with `briefcase dev` gives the errors shown in the log listing below.

Attempting to navigate the presentation with the left and right arrow keys then doesn't work.

### Steps to reproduce

1. Clone on Windows
2. Run `briefcase dev`

### Expected behavior

Presentation can be navigated with the left and right arrow keys

### Screenshots

_No response_

### Environment

- Operating System: Windows
- Python version: 3.12.10
- Software versions:
- Briefcase: 0.3.25
- Toga: 0.5.1 (according to the current pyproject.toml)

### Logs

```
WARNING: invalid shortcut '>: The value of argument 'value' (39) is invalid for Enum type 'Keys'.

Parameter name: value

at System.Windows.Forms.ToolStripMenuItem.set_ShortcutKeys(Keys value)
WARNING: invalid shortcut '>: The value of argument 'value' (37) is invalid for Enum type 'Keys'.

Parameter name: value

at System.Windows.Forms.ToolStripMenuItem.set_ShortcutKeys(Keys value)
WARNING: invalid shortcut '>: The value of argument 'value' (36) is invalid for Enum type 'Keys'.

Parameter name: value

at System.Windows.Forms.ToolStripMenuItem.set_ShortcutKeys(Keys value)
WARNING: invalid shortcut '>: The value of argument 'value' (35) is invalid for Enum type 'Keys'.

Parameter name: value

at System.Windows.Forms.ToolStripMenuItem.set_ShortcutKeys(Keys value)
Executing took 0.110 seconds
```

### Additional context

_No response_

Reproduce the issue on Windows with `briefcase dev` and inspect the `toga_winforms/app.py` entry point shown in the `App._startup()` log. Trace the invalid LEFT, RIGHT, HOME, and END shortcuts and verify that the arrow keys navigate the presentation without those warnings.

Written by the indexing model from the issue text.