feat(tui): SelectList +description/+scroll/+digit-jump/+page-nav

Enrich the generic select widget with features the upcoming overlays
(permission, model picker, history search) need — instead of each
overlay reinventing its own list logic:

- description field rendered dim/italic alongside the label
- auto-scroll viewport: selection always stays visible; scroll indicator
  (↑ / ↓ / ↕) in the top-right when overflowing
- digit shortcuts 1..9 jump to that 1-based index
- PageUp/PageDown by caller-provided page size; Home/End jump to ends
- set_items / set_selected for external state sync
- render_stateful(&mut self, ...) mutates scroll_top; Widget impl still
  works for read-only callers

23/23 tests pass (10 original + 13 new); clippy clean.

Author: lingcoder