CmdPal: Warn when using a reserved system shortcut without LLKH

### Description of the new feature / enhancement

Show a warning when the user selects a keyboard shortcut that conflicts with a system-reserved one and wouldn’t work because low-level keyboard hooks are not enabled (e.g., Win+Space).

### Scenario when this would be used?

UX

### Supporting information

_No response_

Start by locating CmdPal's keyboard-shortcut selection flow and the handling for low-level keyboard hooks (LLKH). Confirm how reserved shortcuts such as Win+Space are detected, then define the warning behavior for selections that require LLKH; done means the warning appears in that case and does not appear for usable shortcuts.

Written by the indexing model from the issue text.