ui(settings): custom dropdown component replacing native selects

Custom dropdown with:
- Popup positioned via fixed coords (escapes overflow:hidden)
- Checkmark indicator on selected item
- Keyboard navigation (ArrowUp/Down, Enter/Space, Escape)
- Scrollable popup with styled scrollbar
- Hover active state tracking
- Click-outside and Escape to close
- Integrated with applySettings/save via hidden inputs
- data-options JSON attribute for declarative config

Replaces native <select> for searchModeDefault and webSearchProvider.