refactor(viewer): consolidate adapter event handling into BaseAdapter

Extracts duplicated hover and selection handling from CADAdapter and
SplineAdapter into BaseAdapter. Converts private adapter methods to
protected methods across the module.

- Moves `_handle_hover` and `_handle_curve_selected` to `BaseAdapter`
- Adds `_get_hover_info` hook for model-specific HUD info formatting
- Updates type hints and improves pick event handling in `CADAdapter`

Author: Ludofr3