Add click-through support to Always On Top windows

### Description of the new feature / enhancement

Add an optional click-through / mouse passthrough mode for "Always On Top" windows, ideally combined with optional transparency controls.

This would allow users to keep certain windows visible as lightweight overlays while still being able to interact with the application underneath.

### Scenario when this would be used?

This could be useful for many productivity and multitasking scenarios, for example:

monitoring chats or notifications while gaming
keeping reference notes visible while working
monitoring audio levels or stats during streaming
lightweight desktop widgets or overlays
accessibility use cases

Currently, users often rely on third-party tools, AutoHotkey scripts, or older overlay applications to achieve this behavior.

### Supporting information

PowerToys already provides an excellent "Always On Top" feature, so optional click-through support feels like a natural extension of it.

Even a simple implementation without advanced overlay features could already improve usability quite a lot for users who work or multitask with a single monitor setup.

No files, tests, or entry points are identified in the issue. Start by locating the existing Always On Top implementation and its tests, then clarify the supported click-through behavior and whether transparency is in scope. Done should include an optional passthrough mode that preserves the underlying application's interaction, with tests covering the new behavior.

Written by the indexing model from the issue text.