Proposal: Make the default expander icon of TreeView larger

### Title

The default expander icon of TreeView is too small

### Summary
The current expander icon is small and hard to see. We should make it larger, similar to Windows Explorer.

- _Default icon:_

- _Explore icon:_

- _MFC icon:_

### Rationale

None.

### Scope

| Capability | Priority |
| :---------- | :------- |
| This proposal will allow developers to accomplish W | Must |
| This proposal will allow end users to accomplish X | Should |
| This proposal will allow developers to accomplish Y | Could |
| This proposal will allow end users to accomplish Z | Won't |

### Important Notes

_No response_

### Open Questions

_No response_

The issue names the TreeView control but provides no source file, test, or entry point. Start by locating TreeView’s default expander styling and icon assets, then compare the current icon with the reference images. Done means the default expander icon is visibly larger and the change is covered by the project’s available visual or control tests.

Written by the indexing model from the issue text.