Allow application-specific remapping for individual keys in Keyboard Manager

### Description of the new feature / enhancement

Currently, Keyboard Manager allows application-specific remapping for shortcuts, but not for individual keys.

For example, I can configure:

Alt + G → A
Target app: example.exe

However, I cannot configure:

G → A
Target app: example.exe

PLS do sth about it, The Target App option is not available when remapping a single key. (many asked for this option for years!)

### Scenario when this would be used?

Use case

I would like to remap a key only while a specific application is running.

For example:

Normal Windows usage:
G → G

While example.exe is active:
G → A

This would be particularly useful for games and applications that have different key layouts or require application-specific key bindings.

### Supporting information

_No response_

Start in Keyboard Manager's existing application-specific shortcut-remapping behavior and compare it with the individual-key remapping flow. Verify that a target application can be selected for a single-key mapping and that the mapping applies only while that application is active, while global behavior remains unchanged. Add or update coverage for both cases.

Written by the indexing model from the issue text.