* feat(tui): implement completion popup rendering and windowing logic
* feat(tui): enhance completion popup with dynamic row budgeting and CSS adjustments
* Refactor picker widgets to use shared base class for improved code reuse
- Introduced `picker_base.py` to encapsulate common functionality for picker widgets.
- Updated `ModelPickerWidget`, `SkillBrowserWidget`, and `ThreadPickerWidget` to inherit from `PickerWidgetBase`.
- Implemented selection helpers (`first_selectable_index`, `move_selection`) in `picker_base.py` for consistent item navigation.
- Refactored rendering and selection logic in each widget to utilize the new base class methods.
- Added tests for picker functionality to ensure behavior remains consistent post-refactor.
Xi Zhang