Rework `ComboBox`

**Is your feature request related to a problem? Please describe.**
The current implementation of the `ComboBox` doesn't look good. Initially, it was built as a copy of the Material counterpart.

**Describe the solution you'd like**
I'd like to see the `ComboBox` reworked to use the `fluent_ui` built-in `Flyout` implementation, or potentially even `OverlayPortal`. This would bring its appearance and behavior much closer to the actual Windows Fluent design guidelines.

**Describe alternatives you've considered**
The main alternative is keeping the current Material-based implementation, but that defeats the purpose of having a dedicated Fluent UI library.

**Additional context**
Reworking this component will significantly improve the overall fidelity and feel of the library for users relying on dropdown/combo box inputs.

Locate the ComboBox implementation and review how its current Material-based dropdown works. Compare the built-in Flyout and OverlayPortal options, then use the Windows Fluent design guidelines as the target for appearance and behavior. The issue names no files or tests and does not define concrete acceptance criteria.

Written by the indexing model from the issue text.