Migrate the Combo and Drop Down components to signals and modern Angular APIs

[viktorkombov]

Description

The goal is to modernize their internal implementation while preserving the existing public API and component behavior.

The migration should include, where applicable:

• Migrate internal component state to signals/computed state.
• Replace decorator-based queries with signal queries.
• Replace outdated lifecycle/change-detection patterns with modern Angular alternatives.
• Ensure state changes correctly notify Angular in zoneless applications.
• Remove unnecessary manual change detection where signals can handle the update.
• Preserve the existing public API and synchronous behavior.
• Update/add tests to cover the migrated behavior in zoneless mode.

Components

• Combo
• Drop Down

Expected result

The components should use the current Angular reactive primitives internally and work correctly with zoneless change detection without introducing breaking API changes.