[Keyboard Manager] Only apply to a specific app not available for non-chorded key triggers

### Microsoft PowerToys version

0.98.1

### Installation method

Microsoft Store

### Area(s) with issue?

Keyboard Manager

### Steps to reproduce

Enable the new editor
Click Open editor
Add a new remapping

### ✔️ Expected Behavior

I expect to have the checkbox active and be able to successfully apply any trigger to a specific app.

### ❌ Actual Behavior

The box for only apply to a specific app is grayed out. The only way to make it available is to enter a chord into the trigger box. Entering a single key as a target does not enable or grays out the box, if it is enabled.

### Upload Bug Report ZIP-file

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### Additional Information

_No response_

### Other Software

_No response_

Reproduce the issue in Keyboard Manager's new editor using a single-key trigger, following the listed steps. Trace the logic controlling the “Only apply to a specific app” checkbox and compare it with chorded triggers; done means the checkbox is available and the remapping can be limited to a selected app for supported triggers.

Written by the indexing model from the issue text.